LeetCode：22括号生成

class Solution {
    List<String> res = new ArrayList<>();

    public List<String> generateParenthesis(int n) {
        backtrack(new StringBuilder(), 0, 0, n);
        return res;    
    }

    //left代表已经用掉的左括号的数量，right代表用掉的右括号的数量
    private void backtrack(StringBuilder path, int left, int right, int n){
        if(path.length() == n * 2){
            res.add(path.toString());
            return;
        }
        //加左括号
        if(left < n){
            path.append("(");
            backtrack(path, left + 1, right, n);
            path.deleteCharAt(path.length() - 1);
        }
        //加右括号
        if(right < left){
            path.append(")");
            backtrack(path, left, right + 1, n);
            path.deleteCharAt(path.length() - 1);
        }
    }
}

必须在right小于left的时候才能加"）"，不然提前加上会造成结果不合法