fix #5: maxflow is broken when s = t

Author: yosupo06

atcoder/maxflow.hpp

@@ -65,6 +65,7 @@ template <class Cap> struct mf_graph {
     Cap flow(int s, int t, Cap flow_limit) {
         assert(0 <= s && s < _n);
         assert(0 <= t && t < _n);
+        assert(s != t);
 
         std::vector<int> level(_n), iter(_n);
         internal::simple_queue<int> que;

test/unittest/maxflow_test.cpp

@@ -178,3 +178,10 @@ TEST(MaxflowTest, SelfLoop) {
     mf_graph<int>::edge e = {0, 0, 100, 0};
     edge_eq(e, g.get_edge(0));
 }
+
+TEST(MaxflowTest, Invalid) {
+    mf_graph<int> g(2);
+    // https://github.com/atcoder/ac-library/issues/5
+    EXPECT_DEATH(g.flow(0, 0), ".*");
+    EXPECT_DEATH(g.flow(0, 0, 0), ".*");
+}