Leetcode#22||Generate Parentheses

public class Solution {
public List<String> generateParenthesis(int n) {
List<String> result = new ArrayList<String>();

if (n <= 0) {
return result;
}

dfs(result, "", n, n);

return result;
}

private void dfs(List<String> result, String s, int left, int right) {
// prune
if (left > right) {
return;
}
// find solution
if (left == 0 && right == 0) {
result.add(s);
return;
}

if (left > 0) {
dfs(result, s + "(", left - 1, right);
}

if (right > 0) {
dfs(result, s + ")", left, right - 1);
}
}
}