Leetcode 162. Find Peak Element

binary search

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

Given a 0-indexed 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] = -∞. In other words, an element is always considered to be strictly greater than a neighbor that is outside the array.

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.

Solution

class Solution {
public:
    int findPeakElement(vector<int>& nums) {
        int start = 0;
        int end = nums.size() - 1;
        
        while (start < end) {
            int mid = (start + end) / 2;
            int mid_next = mid + 1;
            if (nums[mid] < nums[mid_next]) {
                start = mid_next;
            } else {
                end = mid;
            }
        }
        return end;
    }
};