1.题目


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.


2.解决方案


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


思路：因为只需要找到一个最大值，所以这里也可以用二分查找的思想。如果这个值比右边的值小，那么峰值肯定在右侧，所以修改左边的index，如果这个值比右边的大，那么峰值肯定在左侧。注意这里的结束条件，会有两个数，返回大的那个index即可。