Question

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

Difficulty:Easy

Category:Math

Analyze

Solution

// Runtimes: 36ms
class Solution {
 public:
  bool validMountainArray(vector<int>& A) {
    if (A.size() < 3) return false;
    bool has_up = false, has_down = false;
    for (int i = 1; i < A.size(); ++i) {
      if (A[i] > A[i - 1]) {
        if (has_down) return false;
        has_up = true;

      } else if (A[i] < A[i - 1]) {
        if (!has_up) return false;
        has_down = true;
      } else {
        return false;
      }
    }
    return has_up && has_down;
  }
};