Generate Parentheses

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:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

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

    public void backTrack(int n, List<String> ans, StringBuilder str, int open, int closed) {
        if (open >= n || closed >= n) {
            if (n == open && n == closed) {
                ans.add(str.toString());
            } else if (open > n)
                return;
            else if (closed > n)
                return;
        }
        if (closed < open) {
            str.append(")");
            backTrack(n, ans, str, open, closed + 1);
            str.deleteCharAt(str.length() - 1);
        }
        str.append("(");
        backTrack(n, ans, str, open + 1, closed);
        str.deleteCharAt(str.length() - 1);

    }
}