Write a function to remove invalid parentheses from a string.

BFS/DFS: Remove min parentheses to make valid. BFS: try removing 1,2,... until valid. DFS with count: track balance, prune invalid. Return all valid results (BFS finds min removal).

Two-pass: Count extra ( and ); remove in second pass. BFS guarantees minimum.

def remove_invalid(s):
def valid(s): balance=0; [balance:=balance+1 if c=='(' else (balance-1 if c==')' else balance) for c in s]; return balance==0
level = {s}
while level:
valid_set = [x for x in level if valid(x)]
if valid_set: return valid_set
level = {s[:i]+s[i+1:] for s in level for i in range(len(s)) if s[i] in '()'}