POJ 1273

最大流基础题。基本的思路是：先找增广路，路上的流量减掉，反流量（名字忘记了）加上，直到找不到增广路。

 

#include <cstdio>
#include <cstring>
#include <queue>
#include <climits>

using std::queue;
using std::memset;

const int MAX = 400;
int flow[MAX][MAX];
int father[MAX];
bool visit[MAX];
int n,m;

int ford_fulkerson()
{
    int ans = 0;
    queue<int> q;

    while(1)
    {
        while(!q.empty()) q.pop();

        q.push(1);

        memset(visit, 0, sizeof(visit));
        memset(father, 0, sizeof(father));

        while(!q.empty())
        {
            int from = q.front();
            q.pop();

            if(from == n)
            break;

            for(int to = 1 ; to <= n; to++)
            {
                if(flow[from][to] && !visit[to])
                {
                    father[to] = from;
                    q.push(to);
                    visit[to] = true;
                }
            }
        }

        if(!visit[n])
        return ans;

        int minflow = INT_MAX;
        for(int to = n; to != 1; to = father[to])
        {
            int from = father[to];
            if(minflow > flow[from][to])
            minflow = flow[from][to];
        }

        for(int to = n; to != 1; to = father[to])
        {
            int from = father[to];
            flow[from][to] -= minflow;
            flow[to][from] += minflow;
        }
        ans += minflow;
    }
}

int main()
{
    while(scanf("%d%d",&m,&n)!=EOF)
    {
        memset(flow, 0, sizeof(flow));

        for(int i = 0 ; i < m; i++)
        {
            int from, to, val;
            scanf("%d%d%d",&from, &to, &val);
            flow[from][to] += val;
        }
        printf("%d\n", ford_fulkerson());
    }
}