LeetCode(BackTracking)Generate Parentheses

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:

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

思路：自己没想出来，看的别人的代码，只是能看懂，并不能自己想明白其中的一些细节，先mark一下，这道题很难。

代码如下：

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res=new ArrayList<String>();
        if(n<=0){
            return res;
        }
        String str="";
        helper(res,str,0,0,n);
        return res;
        
    }
    public void helper(List<String> res,String str,int open,int close,int max){
        if(str.length()==max*2){
            res.add(str);
            return;
        }
        
        if(open<max){
            helper(res,str+"(",open+1,close,max);
        }
        if(close<open){
            helper(res,str+")",open,close+1,max);
        }
    }
}