Valid Mountain Array

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[A.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 

思路：就是按照定義來，注意開頭和結尾特殊判斷一下，中間的用i來記錄最高點；

class Solution {
    public boolean validMountainArray(int[] A) {
        if(A == null || A.length < 3) {
            return false;
        }
        int n = A.length;
        // 開頭和結尾特殊判斷，然後判斷中間；
        if(A[0] > A[1] || A[n - 1] > A[n - 2]) {
            return false;
        } 
        
        int i = 1;
        for(; i < n; i++) {
            if(A[i - 1] == A[i]) {
                return false;
            } 
            if(A[i - 1] > A[i]) {
                break;
            }
        }

        for(int j = i; j < n - 1; j++) {
            if(A[j] <= A[j + 1]) {
                return false;
            }
        }
        return true;
    }
}