Question

How can I do a replace on a string so that “(” becomes “)” and “)” becomes “(“?

Answer 1

It would be tempting to use .replace() but if you replace ) with ( and then ( with ) you’ll end up with all ). Instead, iterate over the string and use a stringbuilder to build your string.

String swapParens(String s) {
    StringBuilder sb = new StringBuilder();
    sb.ensureCapacity(s.length()); // preallocate to prevent resizing
    for(int i = 0; i < s.length(); i++) {
        char c = s.charAt(i);
        switch(c) {
            case ')': sb.append('('); break;
            case '(': sb.append(')'); break;
            default: sb.append(c);
        }
    }
    return sb.toString();

}

I’m aware you could put something in there as a placeholder to swap against, but if it already existed in your string, you’d have a big problem.

Consider using “xxx” as your swap string. If your string was “abcx(yz)”, and you replace ( with xxx, you end up with “abcxxxyz)” Then you replace ) with ( so you have “abcxxxxyz(“. Then you replace xxx with ) so you have “abc)xyz(“. Certainly not cool!