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.

二分查找！

1 class Solution { 2 public: 3     int findPeak(const vector
     
       &num, int left, int right) { 4         if (left == right) return left; 5         int mid1 = (left + right) / 2; 6         int mid2 = mid1 + 1; 7         if (num[mid1] > num[mid2]) { 8             return findPeak(num, left, mid1); 9         } else {10             return findPeak(num, mid2, right);11         }12     }13     int findPeakElement(const vector
      
        &num) {14         return findPeak(num, 0, num.size() - 1);15     }16 };