def init_graph():
"""
功能：初始化图，确定流的源点，宿点，初始化流为0
输入：无
输出：graph（有向图） sour（源点） dest（宿点）
"""
m = open('outgraph_4.txt', 'r') # 创建图
text = m.readlines()
node_num, edge_num, sour, dest = text.pop(0).split()
graph = Digraph.Graph(node_num, edge_num)
for line in text:
a, b, w = line.split()
w = int(w)
graph.insert(a, b, w)
m.close()
return graph, sour, dest
