LeetCode-in-Java

162. Find Peak Element

Medium

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

Given an integer array nums, find a peak element, and return its index. If the array contains multiple peaks, return the index to any of the peaks.

You may imagine that nums[-1] = nums[n] = -∞.

You must write an algorithm that runs in O(log n) time.

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: 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.

Constraints:

• 1 <= nums.length <= 1000
• -231 <= nums[i] <= 231 - 1
• nums[i] != nums[i + 1] for all valid i.

Solution

public class Solution {
    public int findPeakElement(int[] nums) {
        int start = 0;
        int end = nums.length - 1;
        while (start < end) {
            // This is done because start and end might be big numbers, so it might exceed the
            // integer limit.
            int mid = start + ((end - start) / 2);
            if (nums[mid + 1] > nums[mid]) {
                start = mid + 1;
            } else {
                end = mid;
            }
        }
        return start;
    }
}

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