The problem can be solved by actually constructing the \(N^K\) strings and sorting them.

It is easy to enumerate the \(N^K\) strings using DFS (Depth-First Search).

Writer’ solution (Python)

N,K,X=map(int,input().split())
S=[input() for _ in range(N)]

ans=[]

def dfs(crr, count):
  # Now, `count` strings have been concatenating, resulting in the string `crr`
  if count==K:
    ans.append(crr)
    return
  for s in S:
    dfs(crr+s, count+1)

dfs("", 0)
ans.sort()
print(ans[X-1])