LeetCode Find Peak Element

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。

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