[Leetcode] 845. Longest Mountain in Array (Python)

题目大意

先up再down找序列中最长山脉的长度。

Let’s call any (contiguous) subarray B (of A) a mountain if the following properties hold:

B.length >= 3
There exists some 0 < i < B.length - 1 such that B[0] < B[1] < … B[i-1] < B[i] > B[i+1] > … > B[B.length - 1]
(Note that B could be any subarray of A, including the entire array A.)

Given an array A of integers, return the length of the longest mountain.
Return 0 if there is no mountain.

Example 1:
Input: [2,1,4,7,3,2,5]
Output: 5
Explanation: The largest mountain is [1,4,7,3,2] which has length 5.

Example 2:
Input: [2,2,2]
Output: 0
Explanation: There is no mountain.

题目解析

遍历一次，采用计数的方法。
如果遇到up的时候，lastup计数+1
如果遇到down的时候，更新变量确定前面部分是否有up，lastdown计数+1，如果前面已经有山脉的up子序列，则已经构成山脉，更新ans。

代码

class Solution:
    def longestMountain(self, A: List[int]) -> int:
        lastup = lastdown = ans = sub = 0
        for i in range(1, len(A)):
            if A[i]-A[i-1] > 0:
                lastdown = sub = 0
                lastup += 1
            elif A[i]-A[i-1] < 0:
                sub = max(sub, lastup)
                lastup = 0
                lastdown += 1
                if sub > 0:
                    ans = max(ans, sub+lastdown+1)
            else:
                lastdown = lastup = sub = 0
        return ans