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dinic.py
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74 lines (54 loc) · 1.5 KB
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"""Dinic's algorithm for maximum flow using BFS levels and DFS blocking flows."""
from collections import deque
def add_edge(E, adj, v, u, cap):
i = len(E)
E.append([v, u, cap, 0])
E.append([u, v, 0, 0])
adj[v].append(i)
adj[u].append(i + 1)
def bfs(n, adj, E, s):
level = [-1] * n
level[s] = 0
q = deque([s])
while q:
v = q.popleft()
for i in adj[v]:
_, u, cap, flow = E[i]
if flow == cap or level[u] != -1:
continue
level[u] = level[v] + 1
q.append(u)
return level
def dfs(v, t, pushed, adj, E, level, ptr):
if pushed == 0 or v == t:
return pushed
while ptr[v] < len(adj[v]):
i = adj[v][ptr[v]]
_, u, cap, flow = E[i]
if level[u] != level[v] + 1:
ptr[v] += 1
continue
tr = dfs(u, t, min(pushed, cap - flow), adj, E, level, ptr)
if tr == 0:
ptr[v] += 1
continue
E[i][3] += tr
E[i ^ 1][3] -= tr
return tr
return 0
def dinic(n, edges, s, t):
E = []
adj = [[] for _ in range(n)]
for v, u, cap in edges:
add_edge(E, adj, v, u, cap)
flow = 0
INF = 10**18
while True:
level = bfs(n, adj, E, s)
if level[t] == -1:
return flow
ptr = [0] * n
pushed = dfs(s, t, INF, adj, E, level, ptr)
while pushed:
flow += pushed
pushed = dfs(s, t, INF, adj, E, level, ptr)