941 Valid Mountain Array 有效的山脉数组
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[A.length - 1]
Mountain Array
Example:
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
题目描述:
给定一个整数数组 A，如果它是有效的山脉数组就返回 true，否则返回 false。
让我们回顾一下，如果 A 满足下述条件，那么它是一个山脉数组：
A.length >= 3
在 0 < i < A.length - 1 条件下，存在 I 使得：
A[0] < A[1] < ... A[i-1] < A[I]
A[i] > A[i+1] > ... > A[B.length - 1]
示例 :
示例 1：
输入：[2,1]
输出：false
示例 2：
输入：[3,5,5]
输出：false
示例 3：
输入：[0,3,2,1]
输出：true
提示：
0 <= A.length <= 10000
0 <= A[i] <= 10000
思路:
双指针法
时间复杂度O(n), 空间复杂度O(1)
代码:
C++:
class Solution
{
public:
bool validMountainArray(vector& A)
{
int i = 0, j = A.size() - 1;
while (i < j and A[i] < A[i + 1]) ++i;
while (i < j and A[j] < A[j - 1]) --j;
return i == j and i and j != (A.size() - 1);
}
};
Java:
class Solution {
public boolean validMountainArray(int[] A) {
int i = 0, j = A.length - 1;
while (i < j && A[i] < A[i + 1]) i++;
while (i < j && A[j] < A[j - 1]) j--;
return i == j && i != 0 && j != (A.length - 1);
}
}
Python:
class Solution:
def validMountainArray(self, A: List[int]) -> bool:
i, j = 0, len(A) - 1
while i < j and A[i] < A[i + 1]:
i += 1
while i < j and A[j] < A[j - 1]:
j -= 1
return i == j and not not i and j != len(A) - 1