LeetCode 22. 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:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

public class Solution {
    private List<String> list = new ArrayList<String>();  

    public List<String> generateParenthesis(int n) {
        run("", n, 0);
        return list;
    }  

    // l 为剩余左括号数，r为剩余未匹配的右括号数目
    public void run(String str, int l, int r) {
        if (l == 0 && r == 0) {
            list.add(str);
            return;  //跳出run方法
        }
        if (l > 0) {
            String s = str + "(";
            run(s, l-1, r+1);
        }
        if (r > 0) {
            String s = str + ")";
            run(s, l, r-1);
        }
    }
}  

程序结果：