题目描述：

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.

代码：

int findPeak(const vector<int> &num,int start,int end)
{
    if(start == end)
        return start;
    else if(end - start == 1)
    {
        if(num[end] > num[start])
            return end;
        else
            return start;
    }
    else
    {
        int mid = (start+end)/2;
        if(num[mid] > num[mid+1] && num[mid] > num[mid-1])
            return mid;
        else if(num[mid] > num[mid+1] && num[mid-1] > num[mid])
            return findPeak(num,mid+1,end);
        else
            return findPeak(num,start,mid-1);
    }
}

int Solution::findPeakElement(const vector<int> &num)
{
        return findPeak(num,0,num.size()-1);
}

LeetCode:Find Peak Element