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。

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