LeetCode 941 - Valid Mountain Array

https://leetcode.com/problems/valid-mountain-array/

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

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

• A.length >= 3
• There exists some i with 0 < i < A.length - 1 such that:
  • A[0] < A[1] < ... A[i-1] < A[i]
  • A[i] > A[i+1] > ... > A[B.length - 1]

Example 1:

Input: [2,1]
Output: false

Example 2:

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

Example 3:

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

Note:

1. 0 <= A.length <= 10000
2. 0 <= A[i] <= 10000 

If we walk along the mountain from left to right, we have to move strictly up, then strictly down.

Algorithm

Let's walk up from left to right until we can't: that has to be the peak. We should ensure the peak is not the first or last element. Then, we walk down. If we reach the end, the array is valid, otherwise its not.

  public boolean validMountainArray(int[] A) {

    int N = A.length;

    int i = 0;

    // walk up

    while (i + 1 < N && A[i] < A[i + 1])

      i++;

    // peak can't be first or last

    if (i == 0 || i == N - 1)

      return false;

    // walk down

    while (i + 1 < N && A[i] > A[i + 1])

      i++;

    return i == N - 1;

  }

https://leetcode.com/problems/valid-mountain-array/discuss/194900/C%2B%2BJavaPython-Climb-Mountain

Two people climb from left and from right separately.
If they are climbing the same mountain,
they will meet at the same point.

    public boolean validMountainArray(int[] A) {
        int n = A.length, i = 0, j = n - 1;
        while (i + 1 < n && A[i] < A[i + 1]) i++;
        while (j > 0 && A[j - 1] > A[j]) j--;
        return i > 0 && i == j && j < n - 1;
    }