HDU 3549 —— Flow Problem

原题：http://acm.hdu.edu.cn/showproblem.php?pid=3549

题意：求 1 到 n 的最大流，模板题；

#include<cstdio>#include<cstring>#include<string>#include<queue>#include<algorithm>#include<vector>#define inf 0x3f3f3f3fusing namespace std;const int maxn = 1100;int cas, n, m, T = 0;struct Edge{int from, to, cap, flow;}edge[maxn*2];vector<int>G[maxn];int edgenum;void add(int u, int v, int c){edge[edgenum].from = u;edge[edgenum].to = v;edge[edgenum].flow = 0;edge[edgenum].cap = c;edgenum++;edge[edgenum].from = v;edge[edgenum].to = u;edge[edgenum].cap = 0;edge[edgenum].flow = 0;edgenum++;G[u].push_back(edgenum-2);G[v].push_back(edgenum-1);}int d[maxn];bool vis[maxn];void BFS(int s, int t){memset(vis, false, sizeof vis);queue<int>Q;Q.push(t);vis[t] = true;d[t] = 0;while(!Q.empty()){int now = Q.front();Q.pop();for(int i = 0;i<(int)G[now].size();i++){int v = edge[G[now][i]].to;if(!vis[v]){d[v] = d[now] + 1;Q.push(v);vis[v] = true;}}}}int front[maxn];int Augment(int s, int t){int begin = t;int minflow = inf;while(begin != s){Edge& e = edge[front[begin]];minflow = min(minflow, e.cap - e.flow);begin = e.from;}begin = t;while(begin != s){edge[front[begin]].flow += minflow;edge[front[begin]^1].flow -= minflow;begin = edge[front[begin]].from;}return minflow;}int gap[maxn], cur[maxn];int Maxflow(int s, int t){int flow = 0;BFS(s, t);memset(gap, 0, sizeof gap);memset(cur, 0, sizeof cur);for(int i = 1;i<=n;i++)gap[d[i]]++;int top = s;while(d[s] < n){if(top == t){flow += Augment(s, t);top = s;}bool flag = false;for(int i = cur[top];i<(int)G[top].size();i++){Edge& e = edge[G[top][i]];if(e.cap > e.flow && d[e.from] == d[e.to] + 1){flag = true;front[e.to] = G[top][i];cur[top] = i;top = e.to;break;}}if(!flag){int m = n-1;for(int i = 0;i<(int)G[top].size();i++){Edge& e = edge[G[top][i]];if(e.cap > e.flow)m = min(m, d[e.to]);}if(--gap[d[top]] == 0)break;gap[d[top] = m+1]++;cur[top] = 0;if(top != s){top = edge[front[top]].from;}}}return flow;}void init(){for(int i = 1;i<=n;i++)G[i].clear();edgenum = 0;memset(d, 0, sizeof d);}int main(){scanf("%d", &cas);while(cas--){scanf("%d%d", &n, &m);init();while(m--){int u, v, c;scanf("%d%d%d", &u, &v, &c);add(u, v, c);}int ans = Maxflow(1, n);printf("Case %d: %d\n", ++T, ans);}return 0;}