leetcode22.括号生成

这道题目是一道回溯题，特殊之处在于右括号选择时，右括号的当前数量必须少于左括号的数量 

class Solution {

    private List<String> result;

    private void generateParenthesis(int n, StringBuilder stringBuilder, int open, int close) {
        if (2*n == stringBuilder.length()) {
            result.add(stringBuilder.toString());
        }
        if (open < n) {
            stringBuilder.append('(');
            generateParenthesis(n, stringBuilder, open + 1, close);
            stringBuilder.deleteCharAt(stringBuilder.length() - 1);
        }
        if (close < open) {
            stringBuilder.append(')');
            generateParenthesis(n, stringBuilder, open, close + 1);
            stringBuilder.deleteCharAt(stringBuilder.length() - 1);
        }
    }

    public List<String> generateParenthesis(int n) {
        result = new ArrayList<>();
        generateParenthesis(n, new StringBuilder(), 0, 0);
        return result;
    }
}