941. 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] < 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:

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

From: LeetCode
Link: 941. Valid Mountain Array

Solution:

Ideas:

• Walk up while strictly increasing, stop at the peak.

• Peak can’t be at ends.

• Walk down while strictly decreasing.

• If you end exactly at the last index, it’s a valid mountain.

Code:

bool validMountainArray(int* arr, int arrSize){
    if (arrSize < 3) return false;

    int i = 0;

    // climb up (strictly increasing)
    while (i + 1 < arrSize && arr[i] < arr[i + 1]) i++;

    // peak cannot be first or last
    if (i == 0 || i == arrSize - 1) return false;

    // go down (strictly decreasing)
    while (i + 1 < arrSize && arr[i] > arr[i + 1]) i++;

    // must finish exactly at the end
    return i == arrSize - 1;
}