[leetcode] Generate Parentheses

Generate Parentheses

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        if(n>0){
            generate(n,"",0,0,res);
        }
        return res;
    }
    //采用递归树的思想，当左括号数大于右括号数时可以加左或者右括号，否则只能加左括号，当左括号数达到n时，剩下全部加右括号。
    void generate(int n,string s,int lf,int rg,vector<string> &res){
        if(lf==n){//only can add ')'
            res.push_back(s.append(n-rg,')'));
            return;
        }
        generate(n,s+'(',lf+1,rg,res);//add '('
        if(lf>rg){
            generate(n,s+')',lf,rg+1,res);//add ')'
        }
    }
};

class Solution {
public:
    void help(int n,int x,int y,string now,vector<string> &answer){//dfs
        if(y==n){
            answer.push_back(now);
            return;
        }
        if(x<n){
            help(n,x+1,y,now+"(",answer);//加入左括号
        }
        if(x>y){
            help(n,x,y+1,now+")",answer);//加入右括号
        }
    }
    vector<string> generateParenthesis(int n) {
        vector<string> answer;
        help(n,0,0,"",answer);
        return answer;
    }
};

struct node{
    int x,y;
    string now;
};
class Solution {
public:
    void help(int n,vector<string> &answer){//bfs
       if(n==0){
           answer.push_back("");
           return;
       }
       node start,tmp;
       start.x=start.y=0;
       start.now="";
       
       queue<node> q;
       for(q.push(start);!q.empty();q.pop()){
           tmp=q.front();
           node other;
           if(tmp.x<n){
               other.x=tmp.x+1;
               other.y=tmp.y;
               other.now=tmp.now+"(";
               q.push(other);
           }
           if(tmp.x>tmp.y){
               other.x=tmp.x;
               other.y=tmp.y+1;
               other.now=tmp.now+")";
               if(other.y==n){//组合成功
                   answer.push_back(other.now);
               }else{
                   q.push(other);
               }
           }
       }
    }
    vector<string> generateParenthesis(int n) {
        vector<string> answer;
        help(n,answer);
        return answer;
    }
};