leetcode-find peak element

和best time to buy and sell stock中的某个题类似。

思路很简单：在Binary Search上略加修改。规则是：只要mid比其左邻居小，那说明左半段必然存在peak，只访问左半段即可；否则，右半段必然存在peak。需要注意的是，两端也可以是peak。

 

 1 class Solution {
 2 public:
 3     int findPeakElement(const vector<int> &num) {
 4         int n=num.size();
 5         int left=0,right=n-1;
 6         while(left<=right)
 7         {
 8             int mid=left+(right-left)/2;
 9             if((mid==0||num[mid]>num[mid-1])&&(mid==n-1||num[mid]>num[mid+1]))
10                 return mid;
11             if(mid>0&&num[mid-1]>num[mid])//mid>0不要忘记
12                 right=mid-1;
13             else left=mid+1;
14         }       
15     }
16 };