Add LeetCode January 03.

solved/LeetCode/Challenges/2020/January/03.cc

@@ -0,0 +1,43 @@
+#include<bits/stdc++.h>
+
+using namespace std;
+
+bool IsDiv(int a, int b) {
+  if (b == 0) {
+    return false;
+  }
+  return (a % b) == 0;
+}
+
+class Solution {
+  using Mask = bitset<15>;
+  int n;
+  vector<int> seen;
+  int dp(Mask mask) {
+    if (mask.count() == n) {
+      return 1;
+    }
+    if (seen[mask.to_ulong()] != -1) {
+      return seen[mask.to_ulong()];
+    }
+    int ans = 0;
+    int pos = mask.count() + 1;
+    for (int i = 0; i < n; i++) {
+      int val = i + 1;
+      if (mask[i] == 0 && (IsDiv(val, pos) || IsDiv(pos, val))) {
+        Mask next = mask;
+        next.set(i);
+        ans += dp(next);
+      }
+    }
+    seen[mask.to_ulong()] = ans;
+    return ans;
+  }
+
+ public:
+  int countArrangement(int n) {
+    seen.assign((1 << 15) + 10, -1);
+    this->n = n;
+    return dp(Mask());
+  }
+};