LeetCode - Generate Parentheses

GenerateParentheses.java

// Using String
public List<String> generateParenthesis(int n) {
        ArrayList<String> ret = new ArrayList<String>();
        if(n<1) return ret;
        String str = "";
        helper(n,n,ret, str);
        return ret;
    }
    
    private void helper(int left, int right, ArrayList<String> ret, String str){
        if(left==0 && right==0){
            ret.add(str);
        }
        if(left>right) return;
        if(left>0){
            helper(left-1, right, ret, str+"(");
        }
        if(right>0){
            helper(left, right-1, ret, str+")");
        }
    }
    
    // Using StringBuilder
    public List<String> generateParenthesis(int n) {
        ArrayList<String> ret = new ArrayList<>();
        if(n < 0) return ret;
        helper(ret, n, n, new StringBuilder());
        return ret;
    }
    
    public void helper(ArrayList<String> ret, int left, int right, StringBuilder sb){
        if(left == right && left == 0){
            ret.add(sb.toString());
            return;
        }
        
        if(left > 0){
            sb.append('(');
            helper(ret, left-1, right, sb);
            sb.deleteCharAt(sb.length()-1);
        }
        if(right > left){
            sb.append(')');
            helper(ret, left, right-1, sb);
            sb.deleteCharAt(sb.length()-1);
        }
    }