leetcode 162 Find Peak Element(二分法)

Posted 2020-06-16

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.

 题解：二分。从mid开始找，找到了就返回结果。如果mid不是，哪边比mid大，结果必然在那一边的序列里面。

class Solution {
public:
    int findPeakElement(vector<int>& nums) {
        int n=nums.size();
        nums.push_back(numeric_limits<int>::min());
        int l=0,r=n;
        int mid;
        while(l<r){
            mid=(l+r)/2;
            if((mid==0&&nums[mid]>nums[1])||(nums[mid-1]<nums[mid]&&nums[mid+1]<nums[mid])){
                return mid;
            }
            else if(nums[mid-1]>nums[mid]){
                r=mid;
            }
            else{
                l=mid+1;
            }
        }
        return mid;
    }
};