[leetcode] Generate Parenthese

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： 回溯法题目。需要用left和right记录当前需要多少个左括号和右括号，注意当left > right时，说明右括号多了，不能在增加右括号，如此只能返回，回溯，寻找新的分支点。

public class Solution {
public void dfs(ArrayList<String> result, String s, int left, int right){
if(left > right || left < 0 || right < 0){
return;
}
if(left == 0 && right == 0){
result.add(s);
return;
}
dfs(result, s+'(', left - 1, right);
dfs(result, s+')', left, right - 1);
}
public ArrayList<String> generateParenthesis(int n) {
ArrayList<String> result = new ArrayList<String>();
if(n == 0)
return result;
dfs(result, "", n, n);
return result;
}
}