[leetcode] Generate Parentheses

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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:

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

https://oj.leetcode.com/problems/generate-parentheses/

思路1：自己模仿permutation之類的題目，一個一個往資料裡面填，填的時候判斷是否是valid的，每次都要統計括弧數，麻煩。

思路2：改進思路1，也是遞迴實現，將已有的左右括弧數作為參數遞迴下去，寫起來很簡潔。

 

思路1：

public class Solution {public ArrayList<String> generateParenthesis(int n) {ArrayList<String> result = new ArrayList<String>();if (n <= 0)return result;n = n * 2;char str[] = new char[n];generate(0, n, str, result);return result;}private void generate(int cur, int n, char[] s, ArrayList<String> result) {if (cur == n) {result.add(new String(s));} else {int j;int cntL = 0, cntR = 0;for (j = 0; j < cur; j++) {if (s[j] == ‘(‘)cntL++;elsecntR++;}if (cntL > cntR) {s[cur] = ‘)‘;generate(cur + 1, n, s, result);if (cntL < n / 2) {s[cur] = ‘(‘;generate(cur + 1, n, s, result);}} else if (cntL == cntR) {s[cur] = ‘(‘;generate(cur + 1, n, s, result);} else {return;}}}public static void main(String[] args) {System.out.println(new Solution().generateParenthesis(3));}}

思路2：

import java.util.ArrayList;public class Solution {    public ArrayList<String> generateParenthesis(int n) {        ArrayList<String> res = new ArrayList<String>();        if (n <= 0)            return res;        StringBuilder sb = new StringBuilder();        generate(n, n, sb, res);        return res;    }    private void generate(int l, int r, StringBuilder sb, ArrayList<String> res) {        if (r < l)            return;        if (l == 0 && r == 0) {            res.add(sb.toString());        }        if (l > 0) {            sb.append("(");            generate(l - 1, r, sb, res);            sb.deleteCharAt(sb.length() - 1);        }        if (r > 0) {            sb.append(")");            generate(l, r - 1, sb, res);            sb.deleteCharAt(sb.length() - 1);        }    }    public static void main(String[] args) {        System.out.println(new Solution().generateParenthesis(3));    }}

 

 

參考：

http://blog.csdn.net/linhuanmars/article/details/19873463

http://blog.csdn.net/fightforyourdream/article/details/14159435