941. Valid Mountain Array

1. Description

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]

2. Example

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

3. Constraints

• 1 <= arr.length <= $10^4$
• 0 <= arr[i] <= $10^4$

4. Solutions

Loop

n = arr.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    bool validMountainArray(const vector<int> &arr) {
        cin.tie(nullptr);
        std::ios::sync_with_stdio(false);

        int index = 1;
        for (; index < arr.size() && arr[index] > arr[index - 1]; ++index)
            ;

        if (index == 1 || index == arr.size()) {
            return false;
        }

        for (; index < arr.size() && arr[index] < arr[index - 1]; ++index)
            ;

        return index == arr.size();
    }
};