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private ArrayList<String>  ans = new ArrayList<>();
    private void backtrack(int left,int right,StringBuilder sb){
        if(left==0&&right==0){
            ans.add(new String(sb));
            return;
        }

        if(left>right){
            return;
        }

        if(left<0||right<0){
            return;
        }
        sb.append('(');
        backtrack(left-1, right, sb);
        sb.deleteCharAt(sb.length()-1);

        sb.append(')');
        backtrack(left, right-1, sb);
        sb.deleteCharAt(sb.length()-1);
    }
    public ArrayList<String> generateParenthesis(int n) {
        StringBuilder sb = new StringBuilder();
        backtrack(n, n, sb);
        return ans;
    }