fix: correct push-relabel active condition and johnson test expectation

Author: gangs2314

graphs/push_relabel.py

@@ -91,10 +91,7 @@ def max_flow(self, source: int, sink: int) -> int:
             pushed = False
             for v in range(n):
                 if self.push(u, v):
-                    if (
-                        v not in (source, sink)
-                        and self.excess[v] == self.excess[u] + 1
-                    ):
+                    if v not in (source, sink) and self.excess[v] > 0:
                         active.append(v)
                     pushed = True
                     if self.excess[u] == 0:

graphs/tests/test_graph_algorithms.py

@@ -67,7 +67,7 @@ def test_basic_graph(self):
             4: [],
         }
         dist = johnsons_algorithm(graph, 5)
-        assert dist[0][4] == -1  # 0->1->3->4 = -5+3+2 = 0, wait let me check
+        assert dist[0][4] == 0  # 0->1->3->4 = -5+3+2 = 0, wait let me check
         # Actually: 0->1 (-5), 1->2 (4), 2->4 (1) = 0, or 0->1->3->4 = 0
 
     def test_negative_cycle(self):