Leetcode-Generate Parentheses

LiBlog發表於2014-11-21
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:

"((()))", "(()())", "(())()", "()(())", "()()()"

Have you met this question in a real interview?
 
Analysis:
We define the subproblem as if we now have n left parenthese and m right parenthese, how to generate all legal output.
1. We can always put a left parenthese, and then solve the subproblem with n-1 and m.
2. We can put a right parenthese here only if m>n, otherwise, the output violate the mathcing rule. If we put a right parenthese, we then solve the subproblem with n and m-1.
 
Solution:
 1 public class Solution {
 2     public List<String> generateParenthesis(int n) {
 3         List<String> resSet = new ArrayList<String>();
 4         if (n==0) return resSet;
 5         String curStr = "";
 6         parenthesisRecur(curStr,n,n,resSet);
 7         return resSet;
 8     }
 9 
10     public void parenthesisRecur(String curStr, int n, int m, List<String> resSet){
11         if (n==0 && m==0){
12             resSet.add(curStr);
13             return;
14         }
15 
16         if (n>0){
17             curStr = curStr+"(";
18             parenthesisRecur(curStr,n-1,m,resSet);
19             curStr = curStr.substring(0,curStr.length()-1);
20         }
21 
22         if (m>n){
23             curStr=curStr+")";
24             parenthesisRecur(curStr,n,m-1,resSet);
25             curStr = curStr.substring(0,curStr.length()-1);
26         }
27 
28     }
29 }

 

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