532 B | 532 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from datetime import datetime, timedelta
BLANK = " "
# inp = input
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
a = list(input())
b = list(input())
p("".join(sorted(a + b)[:len(a)]))
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()