Leetcode 162: Find Peak Element

A peak element is an element that is greater than its neighbors.

Given an input array where 

num[i] ≠ num[i+1]

, find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine that 

num[-1] = num
= -∞

.

For example, in array 

[1, 2, 3, 1]

, 3 is a peak element and your function should return
the index number 2.
Solution:

Binary search, use the definition of peak element.

Time complexity: O(logn)

Space complexity: O(1)

public class Solution {
public int findPeakElement(int[] nums) {
int start = 0;
int end = nums.length - 1;
int mid;
while (start + 1 < end) {
mid = start + (end - start) / 2;
if (nums[mid] > nums[mid - 1] && nums[mid] > nums[mid + 1]) {
return mid;
} else if (nums[mid] > nums[mid - 1]) {
start = mid;
} else {
end = mid;
}
}
if (nums[start] > nums[end]) {
return start;
} else {
return end;
}
}
}