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Problem: [852. Peak Index in a Mountain Array 山脉数组的峰顶索引](https://leetcode.cn/problems/peak-index-in-a-mountain-array/desc

解题过程

耗时100%，类似二分查找，判断前后的大小，然后缩小比较区间，左侧 前<后，后侧 前 > 后

Code

class Solution { public: int peakIndexInMountainArray(vector<int>& arr) { int l = 0, r = arr.size(), mid; while(l < r) { mid = l + ((r-l)>>1); if(mid-1>=0) { if(arr[mid] > arr[mid-1]) { l = mid + 1; } else { r = mid - 1; } } else if(mid+1 < arr.size()) { if(arr[mid] < arr[mid+1]) { l = mid + 1; } else { r = mid - 1; } } } int mx = arr[l], id = l; if(l-1 >= 0) { if(mx < arr[l-1]) { id = l-1; } } if(l!=r && mx < arr[r]) { id = r; } if(r+1 < arr.size() && mx < arr[r+1]) { id = r+1; } return id; } };