Valid Mountain Array LT941

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 

Idea 1.  遍历验证是否up-down once

Time complexity: O(n)

Space complexity: O(1)

class Solution {
    public boolean validMountainArray(int[] A) {
        int n = A.length;
        // if(n < 3) {
        //     return false;
        // }
        
        int left = 0;
        while(left+1 < n && A[left] < A[left+1]) {
            ++left;
        }
        if(left == 0 || left == n-1) {
            return false;
        }
        
        while(left+1 < n && A[left] > A[left+1]) {
            ++left;
        }
        
        if(left == n-1) {
            return true;
        }
        
        return false;
    }
}

Idea 1. 网上看到的有意思的解法，both two men walking up from the bottom

Time complexity: O(n)

Space complexity: O(1)

class Solution {
    public boolean validMountainArray(int[] A) {
        int n = A.length;
        
        int left = 0;
        while(left+1 < n && A[left] < A[left+1]) {
            ++left;
        }
        
        int right = n-1;
        while(right - 1 >= 0 && A[right-1] > A[right]) {
            --right;
        }
        
        return left == right && left!= 0 && right != n-1;
    }
}