[LeetCode December Challange] Day 10 - Valid Mountain Array

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] < A[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

Time complexity : O(n)
Space complexity : O(1)

class Solution {
public:
    bool validMountainArray(vector<int>& arr) {
        int N = arr.size();
        int idx = 0;
        
        while (idx < N-1 && arr[idx] < arr[idx+1])
            ++idx;
        
        if (idx == 0 || idx == N-1)
            return false;
        
        while (idx < N-1 && arr[idx] > arr[idx+1])
            ++idx;
        
        return idx == N-1;
    }
};