Solution to Leetcode 1021 Remove Outermost Parentheses

In this problem, we are asked to remove the top level parentheses from a string of parentheses, such that (A)(B)(C)(D) becomes ABCD.

Problem statement

The approach I took, was to have a counter, and to add 1 when the character encountered was ( and remove one otherwise (character )).

That way, when the counter is 0, we must encounter ( and remove it from the string. The same happens when the counter is 1 and we encounter ).

For instance the string “(()())(())”, will have the following sequence of counters:

0 (before starting) -> remove the first character, 1, 2, 1, 2, 1 -> and encounter ')', remove character, 0 -> remove character, 1, 2, 1 -> and encountering ')', remove the last character, 0 (after scanning the last character)

The code in Go is the following:

func removeOuterParentheses(S string) string {
    var res bytes.Buffer
    counter := 0
    for _,c := range S {
        if counter != 0 && !(counter == 1 && c == ')') {
            res.WriteRune(c)
        }
        if c == '(' {
            counter++
        } else {
            counter--
        }
    }
    return res.String()
}