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Peak Index in a Mountain Array

Solution 1

Brute Force

/**
 * Question   : 852. Peak Index in a Mountain Array
 * Complexity : Time: O(n) ; Space: O(1)
 * Topics     : Array
 */
class Solution {
    public int peakIndexInMountainArray(int[] arr) {
        if (arr == null || arr.length == 0) {
            return -1;
        }

        for (int i = 0; i < arr.length - 1; i++) {
            if (arr[i] > arr[i + 1]) {
                return i;
            }
        }

        return -1;
    }
}

Solution 2

Binary Search

/**
 * Question   : 852. Peak Index in a Mountain Array
 * Complexity : Time: O(log(n)) ; Space: O(1)
 * Topics     : Array
 */
class Solution {
    public int peakIndexInMountainArray(int[] arr) {
        if (arr == null || arr.length == 0) {
            return -1;
        }

        int low = 0;
        int high = arr.length - 1;

        while (low <= high) {
            int mid = low + (high - low) / 2;

            if ((mid == 0 || arr[mid - 1] < arr[mid]) && (mid == arr.length - 1 || arr[mid] > arr[mid + 1])) {
                return mid;
            } else if (arr[mid] < arr[mid + 1]) {
                low = mid + 1;
            } else {
                high = mid - 1;
            }
        }

        return -1;
    }
}