Generate Parentheses.java

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:

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

public class Solution {
    public ArrayList<String> generateParenthesis(int n) {
        ArrayList<String> ret = new ArrayList<String>();
	StringBuilder sb = new StringBuilder();
	generateParenthesis(n, 0, 0, 0, sb, ret);
	return ret;
    }
	
    public void generateParenthesis(int n, int s, int e, int l, StringBuilder sb, ArrayList<String> ret) {
        if (s == n && e == n) {
            ret.add(sb.toString());
        } else {
            if (l > 0 && e < n) {
        	sb.append(')');
        	generateParenthesis(n, s, e + 1, l - 1, sb, ret);
        	sb.deleteCharAt(sb.length() - 1);
            }
            if (s < n) {
        	sb.append('(');
        	generateParenthesis(n, s + 1, e, l + 1, sb, ret);
        	sb.deleteCharAt(sb.length() - 1);
            }
        }
    }
}