Find Peak Element —— Leetcode

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.

二分法找峰值，代码中有一个注意点。

class Solution {
public:

    int binarySearch(vector<int> &nums, int low, int high) {
        if(low == high)
            return low;
            
        int mid1 = (low+high)/2;
        int mid2 = mid1+1;
        
        if(nums[mid1] < nums[mid2])
            return binarySearch(nums, mid2, high);  //此处若换成mid1则错误，应该选择高的。
        else
            return binarySearch(nums, low, mid1);
    }

    int findPeakElement(vector<int>& nums) {
        return binarySearch(nums, 0, nums.size()-1);
    }
};