Generate Parentheses - LeetCode

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：标准的递归题

犯的错误：

Line16 忘记加上判断条件会导致无限循环

public class Solution {
    public ArrayList<String> generateParenthesis(int n) {
        ArrayList<String> res= new ArrayList<String>();
        String str="";
        generateParanthesis(n,0,0,str,res);
        return res;
    }
    
    public void generateParanthesis(int n, int l, int r, String str, ArrayList<String> res){
        if(r>l){
            return;
        }
        if(str.length()==2*n){
            res.add(str);
        }
        if(l<=n&&r<=n){
            if(r==l){
                generateParanthesis(n,l+1,r,str+"(",res);
            }else{
                if(l<n)
                    generateParanthesis(n,l+1,r,str+"(",res);
                generateParanthesis(n,l,r+1,str+")",res);
            }
        }
    }
}