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]

Constraints

• 1 <= arr.length <= 104
• 0 <= arr[i] <= 104

Solution

The problem Valid Mountain Array can be solved by checking if the given array consists of strictly increasing and strictly decreasing sequences.

Implementation

static const int fast_io = []()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    return 0;
}();

class Solution
{
  public:
    bool validMountainArray(vector<int> &arr)
    {
        int idx = 1;
        int size = arr.size();

        while (idx < size && arr[idx] > arr[idx - 1])
            idx += 1;

        if (idx == 1 || idx == size)
            return false;

        while (idx < size && arr[idx - 1] > arr[idx])
            idx += 1;

        return idx == size;
    }
};