Leetcode 22.生成括号对数

生成括号对数

给出 n 代表生成括号的对数，请你写出一个函数，使其能够生成所有可能的并且有效的括号组合。

例如，给出 n =3，生成结果为：

[

"((()))",

"(()())",

"(())()",

"()(())",

"()()()"

]

 class Solution {
     public List<String> generateParenthesis(int n) {
         List<String> combinations = new ArrayList();
         generateAll(new char[2 * n], 0, combinations);
         return combinations;
     }

     public void generateAll(char[] current, int pos, List<String> result) {
         if (pos == current.length) {
             if (valid(current))
                 result.add(new String(current));
         } else {
             current[pos] = '(';
             generateAll(current, pos+1, result);
             current[pos] = ')';
             generateAll(current, pos+1, result);
         }
     }

     public boolean valid(char[] current) {
         int balance = 0;
         for (char c: current) {
             if (c == '(') balance++;
             else balance--;
             if (balance < 0) return false;
         }
         return (balance == 0);
     }
 }

 class Solution {
     public List<String> generateParenthesis(int n) {
         List<String> ans = new ArrayList();
         backtrack(ans, "", 0, 0, n);
         return ans;
     }

     public void backtrack(List<String> ans, String cur, int open, int close, int max){
         if (cur.length() == max * 2) {
             ans.add(cur);
             return;
         }

         if (open < max)
             backtrack(ans, cur+"(", open+1, close, max);
         if (close < open)
             backtrack(ans, cur+")", open, close+1, max);
     }
 }