941

941. Valid Mountain Array (Easy)

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 

 

 

Companies:
Google

Related Topics:
Array

Solution 1.

// OJ: https://leetcode.com/problems/valid-mountain-array/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    bool validMountainArray(vector<int>& A) {
        int i = 1, N = A.size();
        while (i < N && A[i] > A[i - 1]) ++i;
        if (i == 1 || i == N) return false;
        while (i < N && A[i] < A[i - 1]) ++i;
        return i == N;
    }
};

Solution 2.

A single for loop

// OJ: https://leetcode.com/problems/valid-mountain-array/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    bool validMountainArray(vector<int>& A) {
        int i = 1, N = A.size(), inc = 1;
        for (; i < N; ++i) {
            if (A[i] == A[i - 1] || (!inc && A[i] > A[i - 1])) return false;
            inc = A[i] > A[i - 1];
            if (!inc && i == 1) return false;
        }
        return !inc;
    }
};