[LeetCode]Find Peak Element

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

Given an input array wherenum[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 thatnum[-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.

Credits:Special thanks to@tsfor adding this problem and creating all test cases.

二分法轻松解决

public class Solution {public int findPeakElement(int[] num) {if (num.length == 1)return 0;if (num[0] > num[1])return 0;if (num[num.length – 1] > num[num.length – 2])return num.length – 1;int i = 0, j = num.length – 1;while (i <= j) {int mid = (i + j) / 2;if(mid==0) mid++;if(mid==num.length-1) mid–;if (num[mid] > num[mid – 1] && num[mid] > num[mid + 1]) {return mid;} else if (num[mid] < num[mid + 1]) {i = mid + 1;} else {j = mid – 1;}}throw new IllegalArgumentException();}}

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