[kattis] Add C++ solution for Reactivity Series.

Author: abeaumont

kattis/README.md

@@ -229,6 +229,7 @@ There are solutions for the following
 1. [Racing Around the Alphabet](racingalphabet.cc) ([problem site](https://open.kattis.com/problems/racingalphabet))
 1. [Ragged Right](raggedright.cc) ([problem site](https://open.kattis.com/problems/raggedright))
 1. [Railroad](railroad2.cc) ([problem site](https://open.kattis.com/problems/railroad2))
+1. [Reactivity Series](reactivity.cc) ([problem site](https://open.kattis.com/problems/reactivity))
 1. [Working at the Restaurant](restaurant.cc) ([problem site](https://open.kattis.com/problems/restaurant))
 1. [Reversed Binary Numbers](reversebinary.py) ([problem site](https://open.kattis.com/problems/reversebinary))
 1. [Reverse Rot](reverserot.cc) ([problem site](https://open.kattis.com/problems/reverserot))

kattis/reactivity-1.ans

@@ -0,0 +1 @@
+0 1

kattis/reactivity-1.in

@@ -0,0 +1,2 @@
+2 1
+0 1

kattis/reactivity-2.ans

@@ -0,0 +1 @@
+back to the lab

kattis/reactivity-2.in

@@ -0,0 +1,5 @@
+4 4
+0 1
+0 2
+2 3
+1 3

kattis/reactivity.cc

@@ -0,0 +1,40 @@
+// https://open.kattis.com/problems/reactivity
+#include <bits/stdc++.h>
+
+using namespace std;
+using vi = vector<int>;
+using vvi = vector<vi>;
+using qi = queue<int>;
+
+int main() {
+  int n, m, u, v;
+  cin >> n >> m;
+  vvi g(n);
+  vi a(n), b;
+  for (int i = 0; i < m; i++) {
+    cin >> u >> v;
+    g[u].push_back(v);
+    a[v]++;
+  }
+  qi q;
+  for (int i = 0; i < n; i++)
+    if (!a[i]) q.push(i);
+  bool ok = 1;
+  while (!q.empty()) {
+    if (q.size() > 1) {
+      ok = 0;
+      break;
+    }
+    u = q.front();
+    q.pop();
+    b.push_back(u);
+    for (int v : g[u]) {
+      a[v]--;
+      if (!a[v]) q.push(v);
+    }
+  }
+  if (ok)
+    for (int i = 0; i < n; i++) cout << b[i] << " \n"[i == n - 1];
+  else
+    cout << "back to the lab\n";
+}