852. Peak Index in a Mountain Array

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LeetCode

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Problem

Let’s call an array arr a mountain if the following properties hold:

arr.length >= 3 There exists some i with 0 < i < arr.length - 1 such that: arr[0] < arr[1] < … arr[i-1] < arr[i] arr[i] > arr[i+1] > … > arr[arr.length - 1] Given an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < … arr[i - 1] < arr[i] > arr[i + 1] > … > arr[arr.length - 1].

Example 1:

Input: arr = [0,1,0]
Output: 1

Example 2:

Input: arr = [0,2,1,0]
Output: 1

Example 3:

Input: arr = [0,10,5,2]
Output: 1

Example 4:

Input: arr = [3,4,5,1]
Output: 2

Example 5:

Input: arr = [24,69,100,99,79,78,67,36,26,19]
Output: 2

Code

class Solution {
    public int peakIndexInMountainArray(int[] arr) {
        int left = 0;
        int right = arr.length - 1;
        while (left + 1 < right) {
            int mid = (left + right) / 2;
            if (arr[mid] > arr[mid - 1] && arr[mid] > arr[mid + 1]) return mid;

            if (arr[mid] > arr[mid - 1]) {
                left = mid;
            } else {
                right = mid;
            }
        }

        if (arr[left] > arr[left - 1] && arr[left] > arr[left + 1]) return left;
        return right;
    }
}

class Solution {
    public int peakIndexInMountainArray(int[] arr) {
        for (int i = 1; i < arr.length - 1; i++) {
            if (arr[i] > arr[i - 1] && arr[i] > arr[i + 1]) return i;
        }

        return -1;
    }
}