LeetCode-Valid Mountain Array

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Description：
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:

• 0 <= A.length <= 10000
• 0 <= A[i] <= 10000

题意：判断数组A是否存在小标 $i$，使得 $A[0] &lt; A[1] &lt; ... A[i-1] &lt; A[i]$且 $A[i] &gt; A[i+1] &gt; ... &gt; A[B.length - 1]$；并且 $A$的长度大于或等于3， $0 &lt; i &lt; A.length - 1$；

解法：我们只要分别从首尾遍历，找到第一个不满足大小关系的位置后，判断这两个位置是否相同即可，具体流程如下

1. for st in [0, A.length - 1)
2. until A[st] > A[st + 1]
3. for ed in [A.length - 1, 1]
4. until A[ed] > A[ed - 1]
5. 比较st与ed是否相同，并且满足st与ed的位置在(0, A.length - 1)

Java

class Solution {
    public boolean validMountainArray(int[] A) {
        if (A.length < 3) {
            return false;
        }
        int st = 0;
        int ed = A.length - 1;
        while (st < A.length - 1 && A[st] < A[st + 1]) {
            st++;
        }
        while (ed > 0 && A[ed] < A[ed - 1]) {
            ed--;
        }
        
        return st == ed && st != 0 && st != A.length - 1 ? true : false;
    }
}