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 elements from a and b such that the sum of their values is as closest to target as possible. Return a list of ids of selected elements. If no pair is possible, return an empty list.
Example 1:
Input:
a = [[1, 2], [2, 4], [3, 6]]
b = [[1, 2]]
target = 7
Output: [[2,1]]
Explanation:
There are only three combinations [1, 1], [2, 1], and [3, 1], which have a total sum of 4, 6 and 8, respectively.
Since 6 is the largest sum that does not exceed 7, [2, 1] is the optimal pair.
Example 2:
Input:
a = [[1, 3], [2, 5], [3, 7], [4, 10]]
b = [[1, 2], [2, 3], [3, 4], [4, 5]]
target = 10
Output: [[2, 4], [3, 2]]