How to Find the Longest Mountain in Array in Java

The challenge

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?

The solution in Java code

class Solution {
    public int longestMountain(int[] A) {
        int N = A.length;
        int answer = 0, 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++;

                if (end + 1 < N && A[end] > A[end + 1]) {
                    while (end + 1 < N && A[end] > A[end + 1])
                        end++;
                    answer = Math.max(answer, end - base + 1);
                }
            }

            base = Math.max(end, base + 1);
        }

        return answer;
    }
}
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