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[LeetCode 162] 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
  • -2^31 <= nums[i] <= 2^31 - 1
  • nums[i] != nums[i + 1] for all valid i.

Thoughts

  • Try binary search?
  • Draw a graph for the array?

TypeScript

function findPeakElement(nums: number[]): number {
    let l: number = 0, r = nums.length - 1;
    
    while(l < r){
        let mid: number = Math.floor((l + r)/2);
    
        // we are in decreasing line
        if(nums[mid] > nums[mid + 1]){
            r = mid
        } else {
            l = mid + 1
        }
    }
    return l
};

Reference