题解 | #括号生成#

import java.util.*;


public class Solution {
    /**
     * 
     * @param n int整型 
     * @return string字符串ArrayList
     */
    public ArrayList<String> generateParenthesis (int n) {
    // write code here
    ArrayList<String> result=new ArrayList<>();
    if(n==0){
        return result;
    }
    dfs(n,0,0,0,result,"");
    return result;
}
    public void dfs(int n,int left,int right,int k,ArrayList<String> result,String str){
        if(2*n==k){
            result.add(str);
        }
        if(left<n){
            dfs(n,left+1,right,k+1,result,str+"(");
        }
        if(right<n&&left>right){
            dfs(n,left,right+1,k+1,result,str+")");
        }
    }
}