941. Valid Mountain Array

Problem

Given an array of integers arr, return true if and only if it is a valid mountain array.

Recall that arr is a mountain array if and only if:

• 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]

  Example 1:

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

Example 2:

Input: arr = [3,5,5]
Output: false

Example 3:

Input: arr = [0,3,2,1]
Output: true

  Constraints:

• 1 <= arr.length <= 10^4

• 0 <= arr[i] <= 10^4

Solution (Java)

class Solution {
    public boolean validMountainArray(int[] arr) {
        int i = 0;
        for (; i < arr.length - 1; i++) {
            if (arr[i] == arr[i + 1]) {
                return false;
            } else if (arr[i] > arr[i + 1]) {
                break;
            }
        }
        if (i == 0 || i >= arr.length - 1) {
            return false;
        }
        for (; i < arr.length - 1; i++) {
            if (arr[i] <= arr[i + 1]) {
                return false;
            }
        }
        return i == arr.length - 1;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).