#leetcode#Find Peak Element

Find Peak Element

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Question  Solution 

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.

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Note:

Your solution should be in logarithmic complexity.

知道用binary search，但是一开始思路局限在nums[mid]要和nums[mid - 1], nums[mid + 1] 做比较来看nums[mid] 是否是 peak，这题只要返回任意一个peak就可以了， 这题给出条件 num[i] != num[i + 1], 算是个提示吧， 每次二分只要比较 nums[mid] 和 nums[mid + 1] 的大小关系就可以了

while(l < r){}, 最后返回 l 或者 r 都是可以的，因为while循环结束后 l == r

public class Solution {
    public int findPeakElement(int[] nums) {
        if(nums == null || nums.length == 0)
            return -1;
        int l = 0;
        int r = nums.length - 1;
        while(l < r){
            int mid = l + (r - l) / 2;
            if(nums[mid] > nums[mid + 1]){
                r = mid;
            }else{//nums[mid] < nums[mid + 1]
                l = mid + 1;
            }
        }
        return r;
    }
}