Given a list of integers and a target sum, find all unique pairs of numbers that add up to the target. Return a list of strings where each string represents a pair in the format "num1,num2" with the smaller number first.

Your task is to implement a method that:

  • Accepts a list of integers and a target sum value
  • Finds all unique pairs that sum to the target
  • Returns pairs as formatted strings with the smaller number first
  • Returns an empty list if no pairs are found
  • Returns null if the input list is null

Examples:

Example 1:

List<Integer> numbers = new List<Integer>{1, 2, 3, 4, 5};
Integer target = 5;
List<String> result = findPairsWithSum(numbers, target);
// Returns: ["1,4", "2,3"]

Example 2:

List<Integer> numbers = new List<Integer>{3, 3, 3};
Integer target = 6;
List<String> result = findPairsWithSum(numbers, target);
// Returns: ["3,3"]

Example 3:

List<Integer> numbers = new List<Integer>{1, 2};
Integer target = 10;
List<String> result = findPairsWithSum(numbers, target);
// Returns: []

Example 4:

List<Integer> numbers = null;
Integer target = 5;
List<String> result = findPairsWithSum(numbers, target);
// Returns: null
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