Find Peak Element

Question

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[n] = -∞.

For example, in array [1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.

Requirement: Run in logarithmic complexity.

Analysis:

The pretty straight way is in O(n) running time complexity. Can we do better?

Of Course. Here the trick lies on the fact that the peak element always exist in the original array and can be found by binary search approach.

Solution

public int findPeakElement(int[] nums) {
        if(nums==null || nums.length==0) return -1;
        int left=0, right=nums.length-1;
        while(left<right){
            int mid=left+(right-left)/2;
            if(nums[mid]>nums[mid+1]) right=mid;
            else left=mid+1;
        }
        return left;
    }