网络流量：正确还是错误

Question

In a directed graph with at most one edge between each pair of vertices, if we
replace each directed edge by an undirected edge, the maximum flow 
value remains unchanged.

为什么会错？

为什么以及如何改变流量？

谢谢。

Answer 1

因为边缘可能是错误的方式。还有更多有趣的案例，但请考虑这个微不足道的案例：

S <- T

无论边缘的容量如何，流量均为零。如果你使它无向，那么流动将是边缘的容量。