22 generate parenthesis

https://leetcode.com/problems/generate-parentheses/

挺有意思的，注意终止条件是左右都有n个之后

class Solution {
    private List<String> ans = new ArrayList<>();
    public List<String> generateParenthesis(int n) {
        generate(n, 0, 0, new StringBuilder());
        return ans;
    }
    private void generate(int n, int left, int right, StringBuilder sb) {
        if (left == n && right == n) {
            ans.add(sb.toString());
            return;
        }
        if (left > right) {
            sb.append(')');
            generate(n, left, right + 1, sb);
            sb.deleteCharAt(sb.length() - 1);
        }
        if (left < n) {
            sb.append('(');
            generate(n, left + 1, right, sb);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}