[leetcode] 845. Longest Mountain in Array

Description

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

Follow up:

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

分析

题目的意思是：找出一个数组中的mountain array。

• 大致思路是把每一个位置当成起点，然后先找up上升趋势的值，然后找下降趋势的值，这样就能找到一个mountain array，我们更新res来找到所有mountain array中最长的array的长度。

代码

class Solution {
public:
    int longestMountain(vector<int>& A) {
        int n=A.size();
        int res=0;
        int base=0;
        while(base<n){
            int end=base;
            if(end+1<n&&A[end]<A[end+1]){
                while(end+1<n&&A[end]<A[end+1]) end++;  //up
                if(end+1<n&&A[end]>A[end+1]){
                    while(end+1<n&&A[end]>A[end+1]) end++;  //down
                    res=max(res,end-base+1);
                }
            }
            base=max(base+1,end);
        }
        return res;
    }
};

参考文献

845. Longest Mountain in Array