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.

情況有以下幾種：
If num[i-1] < num[i] > num[i+1], then num[i] is peak
If num[i-1] < num[i] < num[i+1], then num[i+1…n-1] must contains a peak
If num[i-1] > num[i] > num[i+1], then num[0…i-1] must contains a peak
If num[i-1] > num[i] < num[i+1], then both sides have peak
(n is num.length)
遞迴：

public int findPeakElement(int[] num) {    
    return helper(num,0,num.length-1);
}

public int helper(int[] num,int start,int end){
    if(start == end){
        return start;
    }else if(start+1 == end){
        if(num[start] > num[end]) return start;
        return end;
    }else{
        
        int m = (start+end)/2;
        
        if(num[m] > num[m-1] && num[m] > num[m+1]){

            return m;

        }else if(num[m-1] > num[m] && num[m] > num[m+1]){

            return helper(num,start,m-1);

        }else{

            return helper(num,m+1,end);

        }
        
    }
}

迭代：

class Solution {
    public int findPeakElement(int[] nums) {
        int start = 0;
        int end = nums.length - 1;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (nums[mid] > nums[mid - 1]) {
                start = mid;
            } else {
                end = mid;
            }
        }
        return nums[start] >= nums[end] ? start : end; 
    }
}