# Find Peak Element

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.

分析

* **数学性质**：对于任意数组，至少存在一个峰值（因为两端视为 -∞）。

如果左边可以了nums\[m-1]\<nums\[m]，就去找右边：start = m。

二分好像是等于的那头，后判断。

```
class Solution {
    public int findPeakElement(int[] nums) {
        if(nums == null || nums.length == 0){
            return -1;
        }
        int s = 0, e = nums.length - 1;
        int n = nums.length;
        while(s + 1 < e){
            int m = s + (e - s)/2;
            if(nums[m] > nums[m + 1]){
                e = m;
            }else if(nums[m] > nums[m - 1]){
                s = m;
            }else{
                e = m;
            }
        }

        if(nums[s] > nums[e])
            return s;

        return e;

    }
}
```