0845. Longest Mountain in Array (M)

题目

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?

题意

数组中，如果一个区间的左半部分严格单调递增，右半部分严格单调递减，将这个区间称为一个“山”，要求在给定数组中求这样的区间的最大长度。

思路

用变量start、end分别表示这样的区间的两个端点，对于每一个start，找到能使区间成“山”的end值，计算长度，更新start并重复步骤。

代码实现

Java

class Solution {
    public int longestMountain(int[] A) {
        int ans = 0;
        int start = 0, end = 0;
        while (start < A.length) {
            if (end < A.length - 1 && A[end] < A[end + 1]) {
                while (end < A.length - 1 && A[end] < A[end + 1]) {
                    end++;
                }
                if (end < A.length - 1 && A[end] > A[end + 1]) {
                    while (end < A.length - 1 && A[end] > A[end + 1]) {
                        end++;
                    }
                    ans = Math.max(ans, end - start + 1);
                }
            }
            start = end > start ? end : start + 1;
            end = start;
        }
        return ans;
    }
}