[leetcode]Generate Parentheses

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public class Solution { public List<String> generateParenthesis(int n) { if (n < 0) return result; rec(n, 0, ""); return result; } List<String> result = new ArrayList<String>(); private void rec(int available, int unmatch, String fromUpper){ if (available == 0){ for (int i = 0; i < unmatch; i++) fromUpper += ")"; result.add(fromUpper); return; } String addOn = ""; for (int i = 0; i <= unmatch; i++){ rec(available-1, unmatch-i+1, fromUpper+addOn+"("); addOn += ")"; } } }