Solved problem SRM391-D1-500 from TopCoder

Author: AliOsm

TopCoder/README.md

@@ -10,6 +10,7 @@
 - [SRM319-D1-500](http://community.topcoder.com/stat?c=problem_statement&pm=6714&rd=9999)
 - [SRM335-D1-500](https://community.topcoder.com/stat?c=problem_statement&pm=7337&rd=10659)
 - [SRM338-D1-500](https://community.topcoder.com/stat?c=problem_statement&pm=7289&rd=10662)
+- [SRM391-D1-500](https://community.topcoder.com/stat?c=problem_statement&pm=8202&rd=11125)
 - [SRM394-D2-1000](http://community.topcoder.com/stat?c=problem_statement&pm=8547&rd=11128)
 - [SRM458-D2-500](http://community.topcoder.com/stat?c=problem_statement&pm=10726&rd=14180&rm=&cr=14970299)
 - [SRM483-D2-1000](https://community.topcoder.com/stat?c=problem_statement&pm=11073&rd=14236)

TopCoder/SRM391-D1-500.cpp

@@ -0,0 +1,32 @@
+/*
+  Idea:
+    - https://community.topcoder.com/tc?module=Static&d1=match_editorials&d2=srm391
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+long long str[21][21], fact[21];
+
+class KeysInBoxes {
+public:
+  string getAllKeys(int N, int M) {
+    str[0][0] = 1;
+    for(int i = 1; i < 21; ++i)
+      for(int j = 0; j <= i; ++j)
+        str[i][j] = str[i - 1][j - 1] + i * str[i - 1][j];
+
+    fact[0] = 1;
+    for(int i = 1; i < 21; ++i)
+      fact[i] = fact[i - 1] * i;
+
+    long long res = 0;
+    for(int i = 0; i < M; ++i)
+      res += str[N - 1][i];
+
+    long long gcd = __gcd(res, fact[N]);
+
+    return to_string(res / gcd) + "/" + to_string(fact[N] / gcd);
+  }
+};