Problem

In a Meta-style coding screen, you are given a list of integers and a target value. Return all unique pairs of values whose sum equals the target.

A pair should be included only once, regardless of ordering. For example, (1, 5) and (5, 1) represent the same pair. If a value must be used twice, it must appear at least twice in the input.

Formal Specification

• Input: nums — a list of integers, and target — an integer
• Output: a list of unique pairs, where each pair is a list [a, b] such that a + b == target and a <= b
• Return the pairs sorted in ascending order by value.

Examples

Example 1

• Input: nums = [1, 5, 7, -1, 5], target = 6
• Output: [[-1, 7], [1, 5]]
• Explanation: 1 + 5 = 6 and -1 + 7 = 6. The pair [1, 5] appears once even though 5 occurs twice.

Example 2

• Input: nums = [3, 3, 3, 3], target = 6
• Output: [[3, 3]]
• Explanation: The same value can form a valid pair only if it appears at least twice.

Constraints

• 0 <= len(nums) <= 10^5
• -10^9 <= nums[i] <= 10^9
• -10^9 <= target <= 10^9
• Output must contain unique value pairs only