. - 力扣（LeetCode）

class Solution {
public:vector<vector<int>> match;vector<int> vis;int num;void backtrack(int index, int n) {if (index == n + 1) {num++;return;}for (auto &x : match[index]) {if (!vis[x]) {vis[x] = true;backtrack(index + 1, n);vis[x] = false;}}}int countArrangement(int n) {vis.resize(n + 1);match.resize(n + 1);for (int i = 1; i <= n; i++) {for (int j = 1; j <= n; j++) {if (i % j == 0 || j % i == 0) {match[i].push_back(j);}}}backtrack(1, n);return num;}
};