Leetcode 22. Generate Parentheses

Backtracking. 

public class Solution {    public static void backTrack(String current, List<String> ret, int left, int right, int n) {        if (current.length() == n*2) {            ret.add(current);            return;        }        if (left < n)            backTrack(current+"(", ret, left+1, right, n);        if (right < left)            backTrack(current+")", ret, left, right+1, n);    }        public List<String> generateParenthesis(int n) {        List<String> ret = new ArrayList<>();        backTrack("", ret, 0, 0, n);        return ret;    }}