# 845. Longest Mountain in Array 数组中的最长山脉

@TOC

## # 题目描述

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.

Note:

1. 0 <= A.length <= 10000
2. 0 <= A[i] <= 10000

• Can you solve it using only one pass?
• Can you solve it in O(1) space?

## # 解题方法

### # 双数组

inc: [1, 1, 2, 3, 1, 1, 2, 1]
dec: [2, 1, 1, 3, 2, 1, 2, 1]

class Solution(object):
def longestMountain(self, A):
"""
:type A: List[int]
:rtype: int
"""
N = len(A)
inc = [1] * N
dec = [1] * N
for i in range(1, N):
if A[i] - A[i - 1] > 0:
inc[i] = inc[i - 1] + 1
for i in range(N - 2, -1, -1):
if A[i] - A[i + 1] > 0:
dec[i] = dec[i + 1] + 1
res = 0
for i in range(1, N - 1):
if inc[i] != 1 and dec[i] != 1:
res = max(res, inc[i] + dec[i] - 1)
return res