LeetCode 22. Generate Parentheses JAVA

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 ArrayList<String> generateParenthesis(int n) {  
        ArrayList<String> res = new ArrayList<String>();  
        backtracking(n, n, "", res);  
        return res;  
    }  
  
    public void  backtracking(int left, int right, String str, ArrayList<String> res) {  
        if (left == 0 && right == 0) {  
            res.add(str);  
            return;  
        }  
        if (left > 0) {  
             backtracking(left - 1, right, str + '(', res);  
        }  
        if (right > left) {  
             backtracking(left, right - 1, str + ')', res);  
        }  
    }  

The idea here is to only add ‘(’ and ‘)’ that we know will guarantee us a solution (instead of adding 1 too many close). Once we add a ‘(’ we will then discard it and try a ‘)’ which can only close a valid ‘(’. Each of these steps are recursively called.

一定是‘（’在前 而且在添加过程中‘（’的个数不能少于‘）’的个数