遇到?"时，将其替换为0"并重复出现，然后将其替换为1"并再次出现.

When you encounter a '?', replace it with a '0' and recur, then replace it with a '1' and recur.

private void permute(String s) { 
    ... 
    if (s.charAt(i) == '?') { 
        permute(<replacing '?' with '0'>);
        permute(<replacing '?' with '1'>); 
    } 
    ...
}