UVA 1658 (费用流)

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求两条不相交从1到n路径的长度和的最小值.

每个点拆成入点出点,流量1费用为边权,拆成的两个点流量为1费用为0,除了源点和汇点流量为2.

#include <iostream>#include <cstdio>#include <cstring>#include <cmath>#include <algorithm>#include <vector>#include <queue>using namespace std;#define INF 11111111#define maxn 2111#define maxm 2111111int n, m;int s, t;struct node {    int u, v, next, cap, flow, cost;}edge[maxm];int head[maxn], cnt;int pre[maxn], dis[maxn];bool vis[maxn];int N;void init () {    memset (head, -1, sizeof head);    cnt = 0;}void add_edge (int u, int v, int cap, int cost) {    edge[cnt].u = u, edge[cnt].v = v, edge[cnt].cap = cap, edge[cnt].flow = 0;    edge[cnt].cost = cost, edge[cnt].next = head[u], head[u] = cnt++;    edge[cnt].u = v, edge[cnt].v = u, edge[cnt].cap = 0, edge[cnt].flow = 0;    edge[cnt].cost = -cost, edge[cnt].next = head[v], head[v] = cnt++;}bool spfa (int s, int t) {    queue <int> q;    for (int i = 0; i < N; i++) {        dis[i] = INF;        vis[i] = 0;        pre[i] = -1;    }    dis[s] = 0;    vis[s] = 1;    q.push (s);    while (!q.empty ()) {        int u = q.front (); q.pop ();        vis[u] = 0;        for (int i = head[u]; i != -1; i = edge[i].next) {            int v = edge[i].v;            if (edge[i].cap > edge[i].flow && dis[v] > dis[u]+edge[i].cost) {                dis[v] = dis[u]+edge[i].cost;                pre[v] = i;                if (!vis[v]) {                    vis[v] = 1;                    q.push (v);                }            }        }    }    if (pre[t] == -1)        return 0;    else        return 1;}int MCMF (int s, int t, int &cost) {    int flow = 0;    cost = 0;    while (spfa (s, t)) {        int Min = INF;        for (int i = pre[t]; i != -1; i = pre[edge[i^1].v]) {            if (Min > edge[i].cap-edge[i].flow) {                Min = edge[i].cap-edge[i].flow;            }        }        for (int i = pre[t]; i != -1; i = pre[edge[i^1].v]) {            edge[i].flow += Min;            edge[i^1].flow -= Min;            cost += edge[i].cost*Min;        }        flow += Min;    }    return flow;}int main () {    //freopen ("in.txt", "r", stdin);    while (scanf ("%d%d", &n, &m) == 2) {        init ();        add_edge (0, 1, 2, 0);        add_edge (2*(n-1), 2*(n-1)+1, 2, 0);        for (int i = 1; i < n-1; i++) {            add_edge (i*2, i*2+1, 1, 0);        }        for (int i = 0; i < m; i++) {            int u, v, w;            scanf ("%d%d%d", &u, &v, &w);            u--, v--;            add_edge (2*u+1, 2*v, 1, w);        }        int cost = 0;        N = 2*n;        s = 0, t = 2*(n-1);        MCMF (s, t, cost);        printf ("%d\n", cost);    }    return 0;}


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