LeetCode 162. Find Peak Element

A peak element is an element that is greater than its neighbors.

Given an input array nums, where nums[i] ≠ nums[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 nums[-1] = nums[n] = – ∞ .

Example 1:

Input: nums = [1,2,3,1]
Output: 2
Explanation: 3 is a peak element and your function should return the index number 2.

Example 2:

Input: nums = [1,2,1,3,5,6,4]
Output: 1 or 5 
Explanation: Your function can return either index number 1 where the peak element is 2, 
             or index number 5 where the peak element is 6.

Note:

Your solution should be in logarithmic complexity.

解析：求数组中的局部峰值，并且要求log的时间复杂度。考虑二分法，确定好mid之后，与mid+1进行比较大小，如果nums[mid] < nums[mid+1]说明峰值在后面，此时令low=mid+1；否则说明峰值在前面。代码如下：

class Solution {
public:
    int findPeakElement(vector<int>& nums) {
        int low = 0;
        int high = nums.size()-1;
        while(low < high)
        {
            int mid = (low + high) / 2;
            if(nums[mid] < nums[mid+1])
                low = mid + 1;
            else
                high = mid;
        }
        return high;
    }
};