LeetCode 21 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:

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

思路：假设当前字串已经有r个"("，t个“)”，则
1若r=n,且t<n时，则只能向该字串中添加")";
2.若r=t,且t<n时，，则只能向该字串中添加"（";
3.若r>t,且t<n时，，则有两种方案向该字串中添加"（"; 复制该字串，向复制的字符串中添加“)”

public class Solution {public List<String> generateParenthesis(int n) {int i, j, k, counter, len;String str;ArrayList<String> result = new ArrayList<String>();if (n < 1)return result;result.add(new String("("));for (i = 1; i < 2 * n; i++) {len = result.size();for (j = 0; j < len; j++) {counter = 0;for (k = 0; k < result.get(j).length(); k++) {if (result.get(j).charAt(k) == '(')counter++;}if(2 * counter ==result.get(j).length()&&counter<n){str = result.get(j) + '(';result.set(j, str);}if (2 * counter > result.get(j).length() && counter < n) {str = result.get(j) + '(';result.add(str);str = result.get(j) + ')';result.set(j, str);}if(counter==n){str = result.get(j) + ')';result.set(j, str);}}}return result;}}

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