我的Python为何3个TLE

P3376 【模板】网络最大流

jon666 @ 2019-11-11 02:32:22

我把当前边优化写成这样感觉也没错啊,建V次层次图,DFS最坏每到达V个边(增广路),删掉1个边,一共E个边,那最坏就是O(V^2E)哪里错了啊。。。

import sys
sys.setrecursionlimit(100000)
from collections import deque

class NetworkFlow:
    def __init__(self,s,t):
        self.s = s
        self.t = t
        self.c = {}
        self.f = {}

    def add_edge(self,u,v,c):
        if None == self.c.get(u):
            self.c[u] = {}
            self.f[u] = {}
        self.c[u][v] = c
        self.f[u][v] = 0

    def get_cap(self,u,v):
        return self.c[u][v] if u in self.c and v in self.c[u] else 0

    def get_residual_cap(self,u,v):
        if u in self.c and v in self.c[u] and self.c[u][v]>0:
            return self.c[u][v] - self.f[u][v]
        if v in self.c and u in self.c[v] and self.c[v][u]>0:
            return self.f[v][u]
        return 0

    def make_residual_edges(self):
        edges = {}
        for u in self.c.keys():
            for v in self.c[u].keys():
                if u not in edges:
                    edges[u] = {}
                if v not in edges:
                    edges[v] = {}
                edges[u][v] = self.get_residual_cap(u,v)
                edges[v][u] = self.get_residual_cap(v,u)
        return edges

    def make_level_edges(self):
        res = self.make_residual_edges()
        q = deque([self.s])
        vis = {self.s}
        level = {}
        while len(q)>0:
            u = q.popleft()
            for v in res[u].keys():
                if res[u][v] > 0 and not v in vis:
                    if u not in level:
                        level[u] = set()
                    level[u].add(v)
                    vis.add(v)
                    q.append(v)
        return level

    def dfs_level(self,level,u,path):
        path.append(u)
        if u == self.t:
            return True
        if u in level:
            for v in list(level[u]):
                if self.dfs_level(level,v,path):
                    return True
                else:
                    level[u].remove(v)
        path.pop()
        return False

    def update_flow_level(self,path,level):
        pf = float('inf')
        for i in range(len(path)-1):
            u,v = path[i],path[i+1]
            pf = min( pf, self.get_residual_cap(u,v) )
        for i in range(len(path)-1):
            u,v = path[i],path[i+1]
            if self.get_cap(u,v) > 0:
                self.f[u][v] += pf
            else:
                self.f[v][u] -= pf
            if self.get_residual_cap(u,v) <= 0:
                level[u].remove(v)

    def dinic(self):
        while True:
            counter = 0
            level = self.make_level_edges()
            while True:
                path = []
                if self.dfs_level(level,self.s,path):
                    counter +=1
                    self.update_flow_level(path,level)
                else:
                    break
            if counter == 0:
                break

    def max_flow_value(self):
        u = self.s
        val = 0
        for v in self.c[u].keys():
            val += self.f[u][v]
        return val

data = []
counter = 0
while True:
    counter += 1  
    try: 
        line = input().strip().split(' ')
        line = [int(i) for i in line]
        if counter == 1:
            s,t = line[2],line[3]
        else:
            data.append((line[0],line[1],line[2]))
    except EOFError:
        break

network = NetworkFlow(s,t)
cache = {}
for u,v,c in data:
    if u==t:
        continue
    if v==s:
        continue
    if u==v:
        continue
    if None == cache.get(u):
        cache[u] = {}
    if cache.get(v) and cache[v].get(u):
        if cache[v][u] > c:
            cache[v][u] -= c
        elif cache[v][u] == c:
            cache[v].pop(u)
        else:
            x = cache[v].pop(u)
            cache[u][v] = c - x
    elif cache.get(u) and cache[u].get(v):
        cache[u][v] += c
    else:
        cache[u][v] = c

for u in cache.keys():
    for v in cache[u].keys():
        network.add_edge(u,v,cache[u][v])

network.dinic()
val = network.max_flow_value()
print(str(val))

by jon666 @ 2019-11-11 02:36:41

其实我更想知道我这样写能不能保住EV^2时间复杂度...


by 陈诺sb @ 2019-11-11 02:37:38

%%%


by 陈诺sb @ 2019-11-11 02:37:50

让我 看看那 @jon666


by 陈诺sb @ 2019-11-11 02:38:00

你还 没睡么 @jon666


by fa_555 @ 2019-11-11 07:07:00

py 常数不是本来就大吗(

我写的 dij 交 python 全 TLE,交 pypy TLE 一半(


by 三氯甲硅烷 @ 2019-11-11 07:09:58

用py3 or pypy


by lsy263 @ 2019-11-11 07:18:52

@jon666 py本来就慢...= =


by yummy @ 2019-11-11 07:37:00

洛谷时间限制本来就不是为Python设计的
大概是物以希为贵罢。北京的白菜运往浙江,便用红头绳系住菜根,倒挂在水果店头,尊为“胶菜”;福建野生着的芦荟,一到北京就请进温室,且美其名曰“龙舌兰”。Python也颇受了这样的优待,不但OJ支持Python,几个管理还为Python的时限操心操心。


by microchip @ 2019-11-11 07:42:49

c++的速度是py10倍……


by KokiNiwa @ 2019-11-11 07:51:32

@yummy 真会引用[捂脸.jpg]


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