Intersection of Two Arrays

Given two arrays, write a function to compute their intersection.

Example 1:

Input: 
nums1 = 
[1,2,2,1]
, nums2 = 
[2,2]
Output: 
[2]

Example 2:

Input: 
nums1 = 
[4,9,5]
, nums2 = 
[9,4,9,8,4]
Output: 
[9,4]

Note:

• Each element in the result must be unique.

• The result can be in any order.

Analysis & Solution

Below are from: https://leetcode.com/problems/intersection-of-two-arrays/discuss/81969/Three-Java-Solutions

@divingboy89

Use two hash sets

Time complexity: O(n)

public class Solution {
    public int[] intersection(int[] nums1, int[] nums2) {
        Set<Integer> set = new HashSet<>();
        Set<Integer> intersect = new HashSet<>();
        for (int i = 0; i < nums1.length; i++) {
            set.add(nums1[i]);
        }
        for (int i = 0; i < nums2.length; i++) {
            if (set.contains(nums2[i])) {
                intersect.add(nums2[i]);
            }
        }
        int[] result = new int[intersect.size()];
        int i = 0;
        for (Integer num : intersect) {
            result[i++] = num;
        }
        return result;
    }
}

Sort both arrays, use two pointers

Time complexity: O(nlogn)

Seems fastest in OJ (2ms)

public class Solution {
    public int[] intersection(int[] nums1, int[] nums2) {
        Set<Integer> set = new HashSet<>();
        Arrays.sort(nums1);
        Arrays.sort(nums2);
        int i = 0;
        int j = 0;
        while (i < nums1.length && j < nums2.length) {
            if (nums1[i] < nums2[j]) {
                i++;
            } else if (nums1[i] > nums2[j]) {
                j++;
            } else {
                set.add(nums1[i]);
                i++;
                j++;
            }
        }
        int[] result = new int[set.size()];
        int k = 0;
        for (Integer num : set) {
            result[k++] = num;
        }
        return result;
    }
}

Binary search

Time complexity: O(nlogn)

public class Solution {
    public int[] intersection(int[] nums1, int[] nums2) {
        Set<Integer> set = new HashSet<>();
        Arrays.sort(nums2);
        for (Integer num : nums1) {
            if (binarySearch(nums2, num)) {
                set.add(num);
            }
        }
        int i = 0;
        int[] result = new int[set.size()];
        for (Integer num : set) {
            result[i++] = num;
        }
        return result;
    }

    public boolean binarySearch(int[] nums, int target) {
        int low = 0;
        int high = nums.length - 1;
        while (low <= high) {
            int mid = low + (high - low) / 2;
            if (nums[mid] == target) {
                return true;
            }
            if (nums[mid] > target) {
                high = mid - 1;
            } else {
                low = mid + 1;
            }
        }
        return false;
    }
}

Java 8 Using Stream

Set<Integer> set = Arrays.stream(nums2).boxed().collect(Collectors.toSet());
return Arrays.stream(nums1).distinct().filter(e -> set.contains(e)).toArray();

Reference

https://leetcode.com/problems/intersection-of-two-arrays/discuss/81969/Three-Java-Solutions