POJ1273 Drainage Ditches（最大流二次修改版）

這次沒看答案，把去年的代碼重新推翻再寫了下，確實感覺到很多可以優化的地方，然後這個算法沒我想象中的那麼難，網絡流24題可以安排上了，兩個版本

版本1：這個版本是用鄰接矩陣，但是後來發現完全可以用前向星

#include <bits/stdc++.h>
using namespace std;
#define limit (1000 + 5)//防止溢出
#define INF 0x3f3f3f3f
#define inf 0x3f3f3f3f3f
#define lowbit(i) i&(-i)//一步兩步
#define EPS 1e-6
#define FASTIO  ios::sync_with_stdio(false);cin.tie(0);
#define ff(a) printf("%d\n",a );
#define pi(a,b) pair<a,b>
#define rep(i, a, b) for(ll i = a; i <= b ; ++i)
#define per(i, a, b) for(ll i = b ; i >= a ; --i)
#define MOD 988244353
#define FOPEN freopen("C:\\Users\\tiany\\CLionProjects\\acm_01\\data.txt", "rt", stdin)
#define FOUT freopen("C:\\Users\\tiany\\CLionProjects\\acm_01\\dabiao.txt", "wt", stdout)
typedef long long ll;
typedef unsigned long long ull;
ll read(){
    ll sign = 1, x = 0;char s = getchar();
    while(s > '9' || s < '0' ){if(s == '-')sign = -1;s = getchar();}
    while(s >= '0' && s <= '9'){x = (x << 3) + (x << 1) + s - '0';s = getchar();}
    return x * sign;
}//快讀
void write(ll x){
    if(x < 0) putchar('-'),x = -x;
    if(x / 10) write(x / 10);
    putchar(x % 10 + '0');
}
int n,m;
int g[limit][limit];
int dist[limit],vis[limit];
int fa[limit];
void init(){
    memset(dist, INF, sizeof(dist));
    memset(vis, 0 , sizeof(vis));
}

int spfa(){
    queue<int>q;
    init();
    vis[1] = 1;
    dist[1] = 0;
    q.push(1);
    fa[n] = -1;
    while (q.size()){
        int u = q.front();
        q.pop();
        vis[u] = 0;
        rep(v ,1,n){
            if(dist[u] + 1 < dist[v] && g[u][v] > 0){
                dist[v] = dist[u] + 1;
                fa[v] = u;
                if(!vis[v]){
                    vis[v] = 1;
                    q.push(v);
                }
            }
        }
    }
    return ~fa[n];
}
int max_flow;
void dinic(){
    while (spfa()){
        int min_flow = INF;
        for(int u = n ; u - 1 ; u = fa[u]){
            min_flow = min(min_flow, g[fa[u]][u]);
        }
        max_flow += min_flow;
        for(int u = n ; u - 1 ; u = fa[u]){
            g[fa[u]][u] -= min_flow;
            g[u][fa[u]] += min_flow;
        }
    }
}
int main() {
#ifdef LOCAL
    FOPEN;
#endif
    while (cin>>m>>n) {
        m = read(), n = read();
        memset(g, 0 , sizeof(g));
        rep(i, 1, m) {
            int x = read(), y = read(), flow = read();
            g[x][y] += flow;
        }
        dinic();
        write(max_flow),putchar('\n');
    }
    return 0;
}

前向星版本：

#include <bits/stdc++.h>
using namespace std;
#define limit (1000000 + 5)//防止溢出
#define INF 0x3f3f3f3f
#define inf 0x3f3f3f3f3f
#define lowbit(i) i&(-i)//一步兩步
#define EPS 1e-6
#define FASTIO  ios::sync_with_stdio(false);cin.tie(0);
#define ff(a) printf("%d\n",a );
#define pi(a,b) pair<a,b>
#define rep(i, a, b) for(ll i = a; i <= b ; ++i)
#define per(i, a, b) for(ll i = b ; i >= a ; --i)
#define MOD 988244353
#define FOPEN freopen("C:\\Users\\tiany\\CLionProjects\\acm_01\\data.txt", "rt", stdin)
#define FOUT freopen("C:\\Users\\tiany\\CLionProjects\\acm_01\\dabiao.txt", "wt", stdout)
typedef long long ll;
typedef unsigned long long ull;
ll read(){
    ll sign = 1, x = 0;char s = getchar();
    while(s > '9' || s < '0' ){if(s == '-')sign = -1;s = getchar();}
    while(s >= '0' && s <= '9'){x = (x << 3) + (x << 1) + s - '0';s = getchar();}
    return x * sign;
}//快讀
void write(ll x){
    if(x < 0) putchar('-'),x = -x;
    if(x / 10) write(x / 10);
    putchar(x % 10 + '0');
}
int n,m,vs,ve;
int head[limit],cnt;
int dist[limit],fa[limit],vis[limit],pre[limit];
struct node{
    int to, next, flow, w;
}edge[limit<<1];
void add(int u, int v, int flow, int w = 1){
    edge[cnt].to = v;
    edge[cnt].flow = flow;
    edge[cnt].w = w;
    edge[cnt].next = head[u];
    head[u] = cnt++;
}
void init(int flag = 0){
    if(flag){
        memset(dist, INF, sizeof(dist));
        memset(vis, 0 , sizeof(vis));
    }else{
        memset(head, -1, sizeof(head));
        cnt = 0;
    }
}
int spfa(){
    queue<int>q;
    init(1);
    dist[vs] = 0;
    vis[vs] = 1;
    q.push(vs);
    pre[ve] = -1;
    while (q.size()){
        int u = q.front();
        q.pop();
        vis[u] = 0;
        for(int i = head[u]; ~i; i = edge[i].next){
            int v = edge[i].to, w = edge[i].w;
            int stream = edge[i].flow;
            if(dist[u] + w < dist[v] && stream > 0){
                dist[v] = dist[u] + w;
                pre[v] = i;
                fa[v] = u;
                if(!vis[v]){
                    vis[v] = 1;
                    q.push(v);
                }
            }
        }
    }
    return ~pre[ve];
}

int max_flow,min_cost;
void dinic(){
    while (spfa()){
        int min_flow = INF;
        for(int i = ve; i != vs; i = fa[i]){
            min_flow = min(min_flow, edge[pre[i]].flow);
        }
        max_flow += min_flow;
        min_cost  += dist[ve] * min_flow;
        for(int i = ve; i != vs; i = fa[i]){
            edge[pre[i]].flow -= min_flow;
            edge[pre[i] ^ 1].flow += min_flow;
        }
    }
}
int main() {
#ifdef LOCAL
    FOPEN;
#endif
    while(cin>>n>>m){
        vs= 1, ve =m;
        init();
        rep(i, 1, n){
            int x= read(),y = read(), floww = read();
            if(~head[x] || edge[head[x]].flow == 0) {
                add(x, y, floww);
                add(y, x, 0);
            }else{
                edge[head[x]].flow += floww;
            }
        }
        max_flow = 0;
        dinic();
        printf("%d\n", max_flow);
    }
    return 0;
}