Longest Subarray with At Most Two Distinct Numbers

Given a sequence of N positive integers, your task is to find the length of the longest contiguous subarray that contains at most two distinct numbers.

If N is 0, the sequence is empty and the answer should be 0.

The problem tests your ability to use a two-pointer (sliding window) technique in order to efficiently solve a subarray/substring problem using a frequency map or alternative methods.

In mathematical terms, if you have an array \(a_1, a_2, \ldots, a_N\), find the maximum \(L\) such that there exists indices \(i\) and \(j\) (with \(1 \leq i \leq j \leq N\)) satisfying \(j - i + 1 = L\) and the subarray \(a_i, a_{i+1}, \ldots, a_j\) contains at most two unique elements.

inputFormat

The first line contains a single integer N (\(0 \leq N \leq 10^5\)), representing the number of elements in the sequence.

If N is greater than 0, the second line contains N space-separated integers which represent the sequence.

outputFormat

Output a single integer -- the length of the longest contiguous subarray that contains at most two distinct numbers.

7
1 2 1 2 3 1 3

4