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