Task 1: Remove duplicates from a linked list

Given the head of a singly linked list (not necessarily sorted), remove duplicate values so that each value appears only once. The solution must keep the first occurrence of each value and preserve the original relative order of the remaining nodes.

• Input: head of a singly linked list where each node has fields val and next.
• Output: The head of the de-duplicated linked list.
• Constraints (example): Number of nodes n up to 10^5; node values fit in a 32-bit signed integer.

Example

• Input: 1 -> 3 -> 1 -> 2 -> 3 -> null
• Output: 1 -> 3 -> 2 -> null

Task 2: Longest subsequence of x that is a substring of y

Given two strings x and y, find one longest string s such that:

1. s is a subsequence of x (characters chosen in order, not necessarily contiguous), and
2. s is a substring of y (contiguous segment of y).

If multiple answers have the same maximum length, you may return any one of them.

• Input: Strings x, y
• Output: A string s meeting the conditions with maximum possible length (or an empty string if no non-empty solution exists).
• Constraints (example): 1 ≤ |x|, |y| ≤ 2000; characters are lowercase English letters.

Example

• x = "abac", y = "zzbaczz"
• One valid output: "bac" (it is a subsequence of x and a substring of y).