4242. 货币兑换
摘要
Title: 4242. 货币兑换
Tag: spfa、求负环
Memory Limit: 64 MB
Time Limit: 1000 ms
Powered by:NEFU AB-IN
4242. 货币兑换
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题意
见原题
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思路
初始点为S，初始值为V，每次到下一个点，钱会变多或变少，问能不能从起点走一圈回来钱变多
首先图中是一定存在能回到S的环的，那么只要出现任意两个点之间出现正环即可，用spfa找正环即可
此时不用所有点都入队，因为图是都联通的
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代码
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48'''
Author: NEFU AB-IN
Date: 2022-04-16 15:31:37
FilePath: \ACM\Acwing\4242.py
LastEditTime: 2022-04-16 15:31:37
'''
from collections import deque
INF = int(1e9)
N = 110
dist, st, cnt = [0] * N, [0] * N, [0] * N
g = [[] for _ in range(N)]
def spfa(s, v):
dist[s] = v
q = deque()
q.appendleft(s)
st[s] = 1
while q:
u = q.pop()
st[u] = 0
for v, r, c in g[u]:
if dist[v] < (dist[u] - c) * r:
dist[v] = (dist[u] - c) * r
if st[v] == 0:
st[v] = 1
q.appendleft(v)
cnt[v] = cnt[u] + 1
if cnt[v] >= n:
return 1
return 0
n, m, s, v = input().split()
n, m, s = map(int, [n, m, s])
v = float(v)
for i in range(m):
a, b, rab, cab, rba, cba = map(float, input().split())
a, b = map(int, [a, b])
g[a].append([b, rab, cab])
g[b].append([a, rba, cba])
if spfa(s, v):
print("YES")
else:
print("NO")