Python中如何实现Ford-Fulkerson算法？

from collections import defaultdict

class Graph:
    def __init__(self):
        self.graph = defaultdict(dict)

    def add_edge(self, u, v, w):
        self.graph[u][v] = w

    def bfs(self, s, t, parent):
        visited = [False] * len(self.graph)
        queue = []
        queue.append(s)
        visited[s] = True

        while queue:
            u = queue.pop(0)
            for v, w in self.graph[u].items():
                if visited[v] == False and w > 0:
                    queue.append(v)
                    visited[v] = True
                    parent[v] = u
                    if v == t:
                        return True
        return False

    def ford_fulkerson(self, source, sink):
        parent = [-1] * len(self.graph)
        max_flow = 0

        while self.bfs(source, sink, parent):
            path_flow = float("Inf")
            s = sink
            while(s != source):
                path_flow = min(path_flow, self.graph[parent[s]][s])
                s = parent[s]

            max_flow += path_flow

            v = sink
            while(v != source):
                u = parent[v]
                self.graph[u][v] -= path_flow
                if v not in self.graph[u]:
                    self.graph[u][v] = 0
                if u not in self.graph[v]:
                    self.graph[v][u] = 0
                self.graph[v][u] += path_flow
                v = parent[v]

        return max_flow

# 示例使用
g = Graph()
g.add_edge('s', 'a', 16)
g.add_edge('s', 'c', 13)
g.add_edge('a', 'c', 10)
g.add_edge('a', 'd', 12)
g.add_edge('c', 'a', 4)
g.add_edge('c', 'd', 14)
g.add_edge('d', 'b', 20)
g.add_edge('b', 'a', 9)
g.add_edge('b', 't', 20)
g.add_edge('d', 't', 4)

print("最大流量:", g.ford_fulkerson('s', 't'))