Problem link: https://www.codechef.com/problems/LUCKY10

Problem in short:

You are given two strings A and B consisting of digits 0 to 9 and of length n. You create a string C from A and B as follows:

• Re-order the strings A and B in any way you want.
• i-th digit of C is given by C[i] = max{A[i], B[i]}.
• Remove all digits from C that are not 4 or 7.

You want to find the string C which is the lexicographically greatest possible string among all the strings that can be obtained from the given strings A and B by the described process.

Constraints:

• n <= 20,000
• Number of test cases T <= 10,000
• Sum of n over all test cases <= 200,000