LintCode & LeetCode
  • Introduction
  • Linked List
    • Sort List
    • Merge Two Sorted Lists
    • Merge k Sorted Lists
    • Linked List Cycle
    • Linked List Cycle II
    • Add Two Numbers II
    • Add Two Numbers
    • Odd Even Linked List
    • Intersection of Two Linked Lists
    • Reverse Linked List
    • Reverse Linked List II
    • Remove Linked List Elements
    • Remove Nth Node From End of List
    • Middle of the Linked List
    • Design Linked List
      • Design Singly Linked List
      • Design Doubly Linked List
    • Palindrome Linked List
    • Remove Duplicates from Sorted List
    • Remove Duplicates from Sorted List II
    • Implement Stack Using Singly Linked List
    • Copy List with Random Pointer
  • Binary Search
    • Search in Rotated Sorted Array
    • Search in Rotated Sorted Array II
    • Search in a Sorted Array of Unknown Size
    • First Bad Version
    • Find Minimum in Rotated Sorted Array
    • Find Minimum in Rotated Sorted Array II
    • Find Peak Element
    • Search for a Range
    • Find K Closest Elements
    • Search Insert Position
    • Peak Index in a Mountain Array
    • Heaters
  • Hash Table
    • Jewels and Stones
    • Single Number
    • Subdomain Visit Count
    • Design HashMap
    • Design HashSet
    • Logger Rate Limiter
    • Isomorphic Strings
    • Minimum Index Sum of Two Lists
    • Contains Duplicate II
    • Contains Duplicate III
    • Longest Consecutive Sequence
    • Valid Sudoku
    • Distribute Candies
    • Shortest Word Distance
    • Shortest Word Distance II
  • String
    • Rotate String
    • Add Binary
    • Implement strStr()
    • Longest Common Prefix
    • Reverse Words in a String
    • Reverse Words in a String II
    • Reverse Words in a String III
    • Valid Word Abbreviation
    • Group Anagrams
    • Unique Email Addresses
    • Next Closest Time
    • License Key Formatting
    • String to Integer - atoi
    • Ransom Note
    • Multiply Strings
    • Text Justification
    • Reorder Log Files
    • Most Common Word
    • Valid Parenthesis String
    • K-Substring with K different characters
    • Find All Anagrams in a String
    • Find the Closest Palindrome
    • Simplify Path
  • Array
    • Partition Array
    • Median of Two Sorted Arrays
    • Intersection of Two Arrays
    • Intersection of Two Arrays II
    • Maximum Subarray Sum
    • Minimum Subarray Sum
    • Maximum Subarray II
    • Maximum Subarray III
    • Subarray Sum Closest
    • Subarray Sum
    • Plus One
    • Maximum Subarray Difference
    • Maximum Subarray IV
    • Subarray Sum Equals K
    • Intersection of Two Arrays
    • Intersection of Two Arrays II
    • Find Pivot Index
    • Rotate Array
    • Get Smallest Nonnegative Integer Not In The Array
    • Maximize Distance to Closest Person
    • Sort Colors
    • Next Permutation
    • Rotate Image
    • Pour Water
    • Prison Cells After N Days
    • Majority Element
    • Can Place Flowers
    • Candy
  • Matrix
    • Spiral Matrix
    • Set Matrix Zeroes
    • Diagonal Traverse
  • Queue
    • Design Circular Queue
    • Implement Queue using Stacks
    • Implement Queue by Two Stacks
    • Implement Stack using Queues
    • Moving Average from Data Stream
    • Walls and Gates
    • Open the Lock
    • Sliding Window Maximum
    • Implement Queue Using Fixed Length Array
    • Animal Shelter
  • Stack
    • Valid Parentheses
    • Longest Valid Parentheses
    • Min Stack
    • Max Stack
    • Daily Temperatures
    • Evaluate Reverse Polish Notation
    • Next Greater Element I
    • Next Greater Element II
    • Next Greater Element III
    • Largest Rectangle in Histogram
    • Maximal Rectangle
    • Car Fleet
  • Heap
    • Trapping Rain Water II
    • The Skyline Problem
    • Top K Frequent Words
    • Top K Frequent Words II
    • Top K Frequent Elements
    • Top k Largest Numbers
    • Top k Largest Numbers II
    • Minimum Cost to Hire K Workers
    • Kth Largest Element in an Array
    • Kth Smallest Number in Sorted Matrix
    • Kth Smallest Sum In Two Sorted Arrays
    • K Closest Points to the Origin
    • Merge K Sorted Lists
    • Merge K Sorted Arrays
    • Top K Frequent Words - Map Reduce
  • Data Structure & Design
    • Hash Function
    • Heapify
    • LRU Cache
    • LFU Cache
    • Rehashing
    • Stack Sorting
    • Animal Shelter
    • Sliding Window Maximum
    • Moving Average from Data Stream
    • Find Median from Data Stream
    • Sliding Window Median
    • Design Hit Counter
    • Read N Characters Given Read4 II - Call multiple times
    • Read N Characters Given Read4
    • Flatten 2D Vector
    • Flatten Nested List Iterator
    • Design Search Autocomplete System
    • Time Based Key-Value Store
    • Design Tic-Tac-Toe
    • Insert Delete GetRandom O(1)
  • Union Find
    • Find the Connected Component in the Undirected Graph
    • Find the Weak Connected Component in the Directed Graph
    • Graph Valid Tree
    • Number of Islands
    • Number of Islands II
    • Surrounded Regions
    • Most Stones Removed with Same Row or Column
    • Redundant Connection
  • Trie
    • Implement Trie
    • Add and Search Word
    • Word Search II
    • Longest Word in Dictionary
    • Palindrome Pairs
    • Trie Serialization
    • Trie Service
    • Design Search Autocomplete System
    • Typeahead
  • Trees
    • Binary Tree Inorder Traversal
    • Binary Tree Postorder Traversal
    • Binary Tree Preorder Traversal
    • Binary Tree Level Order Traversal
    • Binary Tree Zigzag Level Order Traversal
    • Binary Tree Vertical Order Traversal
    • N-ary Tree Level Order Traversal
    • N-ary Tree Preorder Traversal
    • N-ary Tree Postorder Traversal
    • Construct Binary Tree from Preorder and Inorder Traversal
    • Populating Next Right Pointers in Each Node
    • Populating Next Right Pointers in Each Node II
    • Maximum Depth of Binary Tree
    • Symmetric Tree
    • Validate Binary Search Tree
    • Convert Sorted Array to Binary Search Tree
    • Path Sum
    • Path Sum II
    • Path Sum III
    • Binary Tree Maximum Path Sum
    • Kth Smallest Element in a BST
    • Same Tree
    • Lowest Common Ancestor of a Binary Tree
    • Lowest Common Ancestor of a Binary Search Tree
    • Nested List Weight Sum II
    • BST Node Distance
    • Minimum Distance (Difference) Between BST Nodes
    • Closet Common Manager
    • N-ary Tree Postorder Traversal
    • Serialize and Deserialize Binary Tree
    • Serialize and Deserialize N-ary Tree
    • Diameter of a Binary Tree
    • Print Binary Trees
  • Segment Tree
    • Segment Tree Build
    • Range Sum Query - Mutable
  • Binary Indexed Tree
  • Graph & Search
    • Clone Graph
    • N Queens
    • Six Degrees
    • Number of Islands
    • Number of Distinct Islands
    • Word Search
    • Course Schedule
    • Course Schedule II
    • Word Ladder
    • Redundant Connection
    • Redundant Connection II
    • Longest Increasing Path in a Matrix
    • Reconstruct Itinerary
    • The Maze
    • The Maze II
    • The Maze III
    • Topological Sorting
    • Island Perimeter
    • Flood Fill
    • Cheapest Flights Within K Stops
    • Evaluate Division
    • Alien Dictionary
    • Cut Off Trees for Golf Event
    • Jump Game II
    • Most Stones Removed with Same Row or Column
  • Backtracking
    • Subsets
    • Subsets II
    • Letter Combinations of a Phone Number
    • Permutations
    • Permutations II
    • Combinations
    • Combination Sum
    • Combination Sum II
    • Combination Sum III
    • Combination Sum IV
    • N-Queens
    • N-Queens II
    • Generate Parentheses
    • Subsets of Size K
  • Two Pointers
    • Two Sum II
    • Triangle Count
    • Trapping Rain Water
    • Container with Most Water
    • Minimum Size Subarray Sum
    • Minimum Window Substring
    • Longest Substring Without Repeating Characters
    • Longest Substring with At Most K Distinct Characters
    • Longest Substring with At Most Two Distinct Characters
    • Fruit Into Baskets
    • Nuts & Bolts Problem
    • Valid Palindrome
    • The Smallest Difference
    • Reverse String
    • Remove Element
    • Max Consecutive Ones
    • Max Consecutive Ones II
    • Remove Duplicates from Sorted Array
    • Remove Duplicates from Sorted Array II
    • Move Zeroes
    • Longest Repeating Character Replacement
    • 3Sum With Multiplicity
    • Merge Sorted Array
    • 3Sum Smaller
    • Backspace String Compare
  • Mathematics
    • Ugly Number
    • Ugly Number II
    • Super Ugly Number
    • Sqrt(x)
    • Random Number 1 to 7 With Equal Probability
    • Pow(x, n)
    • Narcissistic Number
    • Rectangle Overlap
    • Happy Number
    • Add N Days to Given Date
    • Reverse Integer
    • Greatest Common Divisor or Highest Common Factor
  • Bit Operation
    • IP to CIDR
  • Random
    • Random Pick with Weight
    • Random Pick Index
    • Linked List Random Node
  • Dynamic Programming
    • House Robber
    • House Robber II
    • House Robber III
    • Longest Increasing Continuous Subsequence
    • Longest Increasing Continuous Subsequence II
    • Coins in a Line
    • Coins in a Line II
    • Coins in a Line III
    • Maximum Product Subarray
    • Longest Palindromic Substring
    • Stone Game
    • Burst Balloons
    • Perfect Squares
    • Triangle
    • Pascal's Triangle
    • Pascal's Triangle II
    • Min Cost Climbing Stairs
    • Climbing Stairs
    • Unique Paths
    • Unique Paths II
    • Minimum Path Sum
    • Word Break
    • Word Break II
    • Range Sum Query - Immutable
    • Decode Ways
    • Edit Distance
    • Unique Binary Search Trees
    • Unique Binary Search Trees II
    • Maximal Rectangle
    • Maximal Square
    • Regular Expression Matching
    • Wildcard Matching
    • Flip Game II
    • Longest Increasing Subsequence
    • Target Sum
    • Partition Equal Subset Sum
    • Coin Change
    • Jump Game
    • Can I Win
    • Maximum Sum Rectangle in a 2D Matrix
    • Cherry Pick
  • Knapsack
    • Backpack
    • Backpack II
    • Backpack III
    • Backpack IV
    • Backpack V
    • Backpack VI
    • Backpack VII
    • Coin Change
    • Coin Change II
  • High Frequency
    • 2 Sum Closest
    • 3 Sum
    • 3 Sum Closest
    • Sort Colors II
    • Majority Number
    • Majority Number II
    • Majority Number III
    • Best Time to Buy and Sell Stock
    • Best Time to Buy and Sell Stock II
    • Best Time to Buy and Sell Stock III
    • Best Time to Buy and Sell Stock IV
    • Two Sum
    • Two Sum II - Input array is sorted
    • Two Sum III - Data structure design
    • Two Sum IV - Input is a BST
    • 4 Sum
    • 4 Sum II
  • Sorting
  • Greedy
    • Jump Game II
    • Remove K Digits
  • Minimax
    • Nim Game
    • Can I Win
  • Sweep Line & Interval
    • Meeting Rooms
    • Meeting Rooms II
    • Merge Intervals
    • Insert Interval
    • Number of Airplanes in the Sky
    • Exam Room
    • Employee Free Time
    • Closest Pair of Points
    • My Calendar I
    • My Calendar II
    • My Calendar III
    • Add Bold Tag in String
  • Other Algorithms and Data Structure
    • Huffman Coding
    • Reservoir Sampling
    • Bloom Filter
    • External Sorting
    • Construct Quad Tree
  • Company Tag
    • Google
      • Guess the Word
      • Raindrop on Sidewalk
    • Airbnb
      • Display Pages (Pagination)
    • Amazon
  • Problem Solving Summary
    • String or Array Rotation
    • Tips for Avoiding Bugs
    • Substring or Subarray Search
    • Sliding Window
    • K Sums
    • Combination Sum Series
    • Knapsack Problems
    • Depth-first Search
    • Large Number Operation
    • Implementation - Simulation
    • Monotonic Stack & Queue
    • Top K Problems
    • Java Interview Tips
      • OOP in Java
      • Conversion in Java
      • Data Structures in Java
    • Algorithm Optimization Tips
  • Reference
Powered by GitBook
On this page
  • Solution & Analysis
  • 主要的注意点和难点
  • 思路:
  • *Using EOF, count, tmp[]
  • Another Implementation
  • Reference

Was this helpful?

  1. Data Structure & Design

Read N Characters Given Read4

Easy

Given a file and assume that you can only read the file using a given method read4, implement a method to read_n_characters.

Method read4:

The API read4reads 4 consecutive characters from the file, then writes those characters into the buffer arraybuf.

The return value is the number of actual characters read.

Note that read4()has its own file pointer, much likeFILE *fpin C.

Definition of read4:

    Parameter:  char[] buf
    Returns:    int

Note: buf[] is destination not source, the results from read4 will be copied to buf[]

Below is a high level example of howread4works:

File file("abcdefghijk"); // File is "abcdefghijk", initially file pointer (fp) points to 'a'
char[] buf = new char[4]; // Create buffer with enough space to store characters
read4(buf); // read4 returns 4. Now buf = "abcd", fp points to 'e'
read4(buf); // read4 returns 4. Now buf = "efgh", fp points to 'i'
read4(buf); // read4 returns 3. Now buf = "ijk", fp points to end of file

Method read:

By using theread4method, implement the method readthat readsncharacters from the file and store it in the buffer array buf. Consider that you cannot manipulate the file directly.

The return value is the number of actual characters read.

Definition of read:

    Parameters:    char[] buf, int n
    Returns:    int

Note: buf[] is destination not source, you will need to write the results to buf[]

Example 1:

Input: 
file = "abc", n = 4

Output: 
3

Explanation:
 After calling your read method, buf should contain "abc". We read a total of 3 characters from the file, so return 3. Note that "abc" is the file's content, not buf. buf is the destination buffer that you will have to write the results to.

Example 2:

Input: 
file = "abcde", n = 5

Output: 
5

Explanation: 
After calling your read method, buf should contain "abcde". We read a total of 5 characters from the file, so return 5.

Example 3:

Input: 
file = "abcdABCD1234", n = 12

Output: 
12

Explanation: 
After calling your read method, buf should contain "abcdABCD1234". We read a total of 12 characters from the file, so return 12.

Example 4:

Input: 
file = "leetcode", n = 5

Output: 
5

Explanation: 
After calling your read method, buf should contain "leetc". We read a total of 5 characters from the file, so return 5.

Note:

  1. Consider that you cannot manipulate the file directly, the file is only accesible for read4 but not for read.

  2. The read function will only be called once for each test case.

  3. You may assume the destination buffer array, buf, is guaranteed to have enough space for storing n characters.

Solution & Analysis

重新理一下这题的 API 和题目描述:

  • 已知 int read4(char[] buf) 可用

  • 你的程序要传入一个 char[] 过去,函数写入最多 4 个 char 在上面,然后返回写入 char 的长度; 到文件末尾的时候返回 char 长度会小于 4.

  • 于是我们的目标是把最终长度为 n 的字符串写入自己函数的 read(char[] buf) 里,然后返回实际长度。

主要的注意点和难点

有的时候 read4 是短板,有的时候 readN 自己是短板(不需要4个那么多的字符),在写入的时候要注意处理下。

可以通过

// get the actual count
count = Math.min(count, n - total);

或者

while (i < count && cur < n) {
...
}

来限制。

思路:

比较巧妙的是用一个pointer cur记录已经从临时buffer tmp[]写完到buf[]的位置,外层循环的条件则是pointer cur < n

EOF的条件则是read4() 返回的count < SIZE, 也就是 count < 4。

/**
 * The read4 API is defined in the parent class Reader4.
 *     int read4(char[] buf);
 */
public class Solution extends Reader4 {
    private int SIZE = 4;
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    public int read(char[] buf, int n) {
        char[] tmp = new char[SIZE];
        int cur = 0;
        int count = 0;
        boolean EOF = false;
        while (!EOF && cur < n) {
            count = read4(tmp);
            EOF = (count < SIZE);

            int i = 0;
            while (i < count && cur < n) {
                buf[cur++] = tmp[i++];
            }
        }

        return cur;
    }
}

*Using EOF, count, tmp[]

public int read(char[] buf, int n) {
  boolean eof = false;      // end of file flag
  int total = 0;            // total bytes have read
  char[] tmp = new char[4]; // temp buffer

  while (!eof && total < n) {
    int count = read4(tmp);

    // check if it's the end of the file
    eof = count < 4;

    // get the actual count
    count = Math.min(count, n - total);

    // copy from temp buffer to buf
    for (int i = 0; i < count; i++) 
      buf[total++] = tmp[i];
  }

  return total;
}

The reason we need Math.min(count, n-total) is because we only want to read N characters even if we have all 4 characters returned from Read4.

Say, N = 18 and we're implentening Read18, then Read4 will takes us to 4*4 = 16 chars. After that, we only want to read 2 more chars (even if the next Read4 returns 3 or 4 characters).

Another Implementation

public int read(char[] buf, int n) {

    char[] temp = new char[4];  //Store our read chars from Read4
    int total = 0;

    while(total < n){
        // Read and store characters in Temp. Count will store total chars read from Read4
        int count = read4(temp);

        // Even if we read 4 chars from Read4, we don't want to exceed N and only want to read chars till N.
        count = Math.min(count, n-total);

        // Transfer all the characters read from Read4 to our buffer
        for(int i = 0;  i < count; i++){
            buf[total] = temp[i];
            total++;
        }

        // EOF. Done. We can't read more characters.
        if(count < 4) break;
    }

    return total;
}

Reference

PreviousRead N Characters Given Read4 II - Call multiple timesNextFlatten 2D Vector

Last updated 5 years ago

Was this helpful?

Source:

https://mnmunknown.gitbooks.io/algorithm-notes/728,_fb_tag_ti.html
https://mnmunknown.gitbooks.io/algorithm-notes/728,_fb_tag_ti.html