✡ leetcode 162. Find Peak Element --------- java

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.

 

找出峰值。

 1、淳朴做法。

public class Solution {
    public int findPeakElement(int[] nums) {
        int len = nums.length;
        
        if( len ==1 || nums[0] > nums[1] )
            return 0;
        if( nums[len-2] < nums[len-1])
            return len-1;
        for( int i = 1 ; i < len-1 ; i++ ){

            if( nums[i] > nums[i-1] && nums[i] > nums[i+1] )
                return i;

            if( nums[i] > nums[i+1] )
                i++;
        }
        return 0;
    }
}

 

2、还可以使用二分查找。（最佳）

 因为从min到max的这个路径中，一定存在一个峰值。

public class Solution {
    public int findPeakElement(int[] nums) {
        for(int min = 0, max = nums.length -1, mid = max / 2 ; min < max ; mid = (min + max) / 2){
            if(min == mid) return nums[min] < nums[max] ? max : min;
            min = nums[mid] < nums[mid+1] ? mid : min;
            max = nums[mid] > nums[mid+1] ? mid : max; 
        }
        return 0;
    }
}