Find Peak Element

Problem

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

/**
 * @param {number[]} nums
 * @return {number}
 */
var findPeakElement = function(nums) {
    let lo = 0;
    let hi = nums.length - 1;
    while (lo <= hi) {
        const mid = Math.floor((lo + hi) / 2);
        if (nums[mid - 1] > nums[mid]) {
            hi = mid;
        } else if (nums[mid] < nums[mid + 1]) {
            lo = mid + 1;
        } else {
            return mid;
        }
    }
};

Use binary search to process the half with the greater neighbor compared to nums[mid]. If no such element exist, nums[mid] is the peak. Notice that we set lo = mid + 1, the reason is although lo could be a peak, we know for certain there is a peak in the index range of [mid + 1, hi], and since we only need to find a single peak, we can discard lo.

Problem​

Solution​

Problem

Solution