22. 括号生成

1.dfs

import java.util.ArrayList;
import java.util.List;

class Solution {
    public List<String> generateParenthesis(int n) {
        char[] p = new char[]{'(', ')'};
        int[] pb = new int[]{n, n};
        List<String> res = new ArrayList<>();
        dfs(n, p, pb, res, "");
        return res;
    }

    public void dfs(int n, char[] p, int[] pb, List<String> res, String str) {
        //1.截至条件
        if (str.length() == 2 * n) {
            res.add(str);
            return;
        }
        //2.候选节点
        if (pb[0] > 0) {
            pb[0]--;
            dfs(n, p, pb, res, str + p[0]);
            pb[0]++;
        }
        if (pb[1] > 0 && pb[0] != pb[1]) {
            pb[1]--;
            dfs(n, p, pb, res, str + p[1]);
            pb[1]++;
        }
    }
}