Longest Mountain in Array

 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?


 class Solution {
    public int longestMountain(int[] A) {
        int result = 0, up = 0, down = 0;
        for (int i = 1; i < A.length; ++i) {
            if (down > 0 && A[i - 1] < A[i] || A[i - 1] == A[i]) up = down = 0;
            if (A[i - 1] < A[i]) up++;
            if (A[i - 1] > A[i]) down++;
            if (up > 0 && down > 0 && up + down + 1 > result) result = up + down + 1;
        }
        return result;
    }
}

In this solution up defines count up mountain length and down defines count down mountain length. Both up and down length are clear to 0 when A[i - 1] == A[i] or down > 0 && A[i - 1] < A[i].

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