Why this solution takes so long?

  • 0

    So I have tried two different approach, the first one is the normal backtracking solution, which takes 2ms.
    And I came up with the solution below.
    The idea is to get the result of one less pair of parentheses, then insert another into each possible location.

    public List<String> generateParenthesis(int n) {
        List<String> rt = new ArrayList();
        if(n == 0){
            return rt;
        List<String> sub = generateParenthesis(n-1);
        for(String item : sub){
            for(int i = 0 ; i <= item.length() / 2; i++){
                // insert another pair
                String newStr = item.substring(0, i) +"()" + item.substring(i);
        return rt;

    It took 25ms. Could anyone help me to understand what's the time complexity here?

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