Add challenge case for #88

Author: MiSawa

src/mincostflow.rs

@@ -209,4 +209,13 @@ mod tests {
         let expected = [(0, 0), (3, 3)];
         assert_eq!(expected[..], *graph.slope(0, 2, i32::max_value()));
     }
+
+    #[test]
+    fn only_one_nonzero_cost_edge() {
+        let mut graph = MinCostFlowGraph::new(3);
+        assert_eq!(0, graph.add_edge(0, 1, 1, 1));
+        assert_eq!(1, graph.add_edge(1, 2, 1, 0));
+        let expected = [(0, 0), (1, 1)];
+        assert_eq!(expected[..], *graph.slope(0, 2, i32::max_value()));
+    }
 }