Peak Index in a Mountain Array
Easy
Let's call an arrayA
amountain if the following properties hold:
A.length >= 3
There exists some
0 < i < A.length - 1
such thatA[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]
Given an array that is definitely a mountain, return any i
such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]
.
Example 1:
Input:
[0,1,0]
Output:
1
Example 2:
Input:
[0,2,1,0]
Output:
1
Note:
3 <= A.length <= 10000
0 <= A[i] <= 10^6
A is a mountain, as defined above.
Solution
Linear Scan
class Solution {
public int peakIndexInMountainArray(int[] A) {
for (int i = 0; i < A.length; i++) {
if (i > 0 && i < A.length - 1 && A[i] > A[i - 1] && A[i] > A[i + 1]) {
return i;
}
}
return -1;
}
}
Linear Scan II
class Solution {
public int peakIndexInMountainArray(int[] A) {
int i = 0;
while (A[i] < A[i+1]) i++;
return i;
}
}
Binary Search
BS Template #2
class Solution {
public int peakIndexInMountainArray(int[] A) {
int left = 0, right = A.length;
while (left < right) {
int mid = left + (right - left) / 2;
if (A[mid] < A[mid + 1]) {
left = mid + 1;
} else {
right = mid;
}
}
return left;
}
}
BS Template #1
class Solution {
public int peakIndexInMountainArray(int[] A) {
int left = 0, right = A.length - 1;
while (left <= right) {
int mid = left + (right - left) / 2;
if (A[mid] < A[mid + 1]) {
left = mid + 1;
} else {
right = mid - 1;
}
}
return left;
}
}
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