POJ 1637 -- Sightseeing tour

#include<cstdio>
#include<cstring>
#include<queue>
#include<iostream>
#include<vector>
#define Min(a,b) ((a)<(b)?(a):(b))
#define Max(a,b) ((a)>(b)?(a):(b))
#define Abs(a) ((a)>(0)?(a):(-a))
using namespace std;
const int N=210, M=2810, INF=0x3f3f3f3f;
int n, m, s, t, top;
int head
, gap
, cur
, dis
, vis
, pre
, in
, out
;

struct Edge{
int to, next, flow;
Edge(int _to = 0, int _next = 0, int _flow = 0):to(_to), next(_next), flow(_flow){}
}edge[M];

void Addedge(int from, int to, int flow){
edge[top] = Edge(to, head[from], flow);
head[from] = top++;
edge[top] = Edge(from, head[to], 0);
head[to] = top++;
}

void Bfs(){
queue<int> q;
memset(gap, 0, sizeof(gap));
memset(dis, -1, sizeof(dis));
gap[0] = 1; dis[t] = 0; q.push(t);
while(!q.empty()){
int u = q.front(); q.pop();
for(int i = head[u]; i+1; i = edge[i].next){
if(dis[edge[i].to] == -1){
dis[edge[i].to] = dis[u] + 1;
gap[dis[edge[i].to]] ++;
q.push(edge[i].to);
}
}
}
}

int Sap(){
Bfs();
memset(pre, -1, sizeof(pre));
for(int i = s; i <= t; ++i) cur[i] = head[i];
int u = s, cur_flow, max_flow = 0, neck, tmp;
while(dis[s] <= t){
if(u == t){
cur_flow = INF;
for(int i = s; i != t; i = edge[cur[i]].to){
if(cur_flow > edge[cur[i]].flow){
cur_flow = edge[cur[i]].flow;
neck = i;
}
}
for(int i = s; i != t; i = edge[cur[i]].to){
tmp = cur[i];
edge[tmp].flow -= cur_flow;
edge[tmp^1].flow += cur_flow;
}
max_flow += cur_flow;
u = neck;
}
int i;
for(i = cur[u]; i + 1; i = edge[i].next){
if(edge[i].flow && dis[u] == dis[edge[i].to] + 1) break;
}
if(i != -1){
cur[u] = i;
pre[edge[i].to] = u;
u = edge[i].to;
}else{
if(--gap[dis[u]] == 0) break;
cur[u] = head[u];
int mindis = t;
for(i = head[u]; i + 1; i = edge[i].next)
if(edge[i].flow && mindis > dis[edge[i].to]) mindis = dis[edge[i].to];
dis[u] = mindis + 1;
gap[dis[u]] ++;
if(u != s) u = pre[u];
}
}
return max_flow;
}

int main(){
int T;
scanf("%d", &T);
while(T--){
scanf("%d%d", &n, &m);
memset(head, -1, sizeof(head));
memset(in, 0, sizeof(in));
memset(out, 0, sizeof(out));
top = s = 0; t = n + 1;
int u, v, d, sum = 0, flag = 0;
for(int i = 1; i <= m; ++i){
scanf("%d%d%d", &u, &v, &d);
in[v]++, out[u]++;
if(d == 0) Addedge(u, v, 1);
}
for(int i = 1; i <= n; ++i){
if(in[i] > out[i]) Addedge(i, t, (in[i]-out[i])/2);
else Addedge(s, i, (out[i]-in[i])/2), sum+=((out[i]-in[i])/2);
if((Abs(in[i]-out[i]))%2) flag = 1;
}
if(flag){
printf("impossible\n");
continue;
}
int res = Sap();
if(res == sum) printf("possible\n");
else printf("impossible\n");
}
}