Given 2 lists `a` and `b`. Each element is a pair of integers where the first integer represents the unique id and the second integer represents a value. Your task is to find an element from `a` and an element form `b` such that the sum of their values is less or equal to `target` and as close to `target` as possible. Return a list of ids of selected elements. If no pair is possible, return an empty list.\n\n**Example 1:**\n```\nInput:\na = [[1, 2], [2, 4], [3, 6]]\nb = [[1, 2]]\ntarget = 7\n\nOutput: [[2, 1]]\n\nExplanation:\nThere are only three combinations [1, 1], [2, 1], and [3, 1], which have a total sum of 4, 6 and 8, respectively.\nSince 6 is the largest sum that does not exceed 7, [2, 1] is the optimal pair.\n```\n\n**Example 2:**\n```\nInput:\na = [[1, 3], [2, 5], [3, 7], [4, 10]]\nb = [[1, 2], [2, 3], [3, 4], [4, 5]]\ntarget = 10\n\nOutput: [[2, 4], [3, 2]]\n\nExplanation:\nThere are two pairs possible. Element with id = 2 from the list `a` has a value 5, and element with id = 4 from the list `b` also has a value 5.\nCombined, they add up to 10. Similarily, element with id = 3 from `a` has a value 7, and element with id = 2 from `b` has a value 3.\nThese also add up to 10. Therefore, the optimal pairs are [2, 4] and [3, 2].\n```\n\n**Example 3:**\n```\nInput:\na = [[1, 8], [2, 7], [3, 14]]\nb = [[1, 5], [2, 10], [3, 14]]\ntarget = 20\n\nOutput: [[3, 1]]\n```\n\n**Example 4:**\n```\nInput:\na = [[1, 8], [2, 15], [3, 9]]\nb = [[1, 8], [2, 11], [3, 12]]\ntarget = 20\n\nOutput: [[1, 3], [3, 2]]\n```

Given 2 lists a and b. Each element is a pair of integers where the first integer represents the unique id and the second integer represents a value. Your task 