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Valid Mountain Array

Updated: Mar 25, 2021

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 <= 104

  • 0 <= arr[i] <= 104

Solution:


class Solution {
    public boolean validMountainArray(int[] arr) {
        int left = 0, right = arr.length-1;
        if(arr.length<3) return false;
        while(left<right && arr[left]<arr[left+1]) 
            left++;
        while(right>=1 && arr[right]<arr[right-1]) 
            right--;
        if(right!=0 && left!=arr.length-1 && left==right) return true;
        return false;
    }
}


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