[LeetCode]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[n] = -∞.

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

LeetCode Source\

Analysis：

This problem is a binary search,but we should do a litte change.

the PeakElement is the largest in the neighbor,when our elment is small than the next one it means that the peak is in the [mid+1,left],else the peak is in the [right,mid].

 class Solution {
public:
    int findPeakElement(const vector<int> &num) {
        int left = 0;
        int right = num.size()-1;
        while(left<right){
            int mid = (right+left)/2;
            if(num[mid]<num[mid+1]){
                left = mid+1;
            }
            else right = mid;
        }
        return right;
    }
};