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;
}
}