leetcode -- Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

典型的递归。一步步构造字符串。当左括号出现次数<n时，就可以放置新的左括号。当右括号出现次数小于左括号出现次数时，就可以放置新的右括号。

public class Solution {
    public ArrayList<String> generateParenthesis(int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<String> result = new ArrayList<String>();
        if(n != 0){
            generate(result, "", 0, 0, 0, n);
        }        
        return result;
    }
    
    public void generate(ArrayList<String> result, String tmp, int deep, int left, int right, int n){
        if(deep == 2 * n){
            result.add(tmp);
            return;
        }
        
        if(left < n){
            tmp += "(";
            generate(result, tmp, deep + 1, left + 1, right, n);
            tmp = tmp.substring(0, tmp.length() - 1);
        }
        
        if(right < left){
            tmp += ")";
            generate(result, tmp, deep + 1, left, right + 1, n);
            tmp = tmp.substring(0, tmp.length() - 1);
        }
        
    }
}