HDU-1532-Drainage Ditches

Posted 2021-02-13 ydddd

链接：https://vjudge.net/problem/HDU-1532

题意：

n条边，m个节点。

求最大流。

多组输入。

思路：

增广路算法。

代码：

#include <iostream>
#include <memory.h>
#include <vector>
#include <map>
#include <algorithm>
#include <cstdio>
#include <math.h>
#include <queue>
#include <string>

using namespace std;

typedef long long LL;

const int MAXN = 200 + 10;
const int INF = 1e9 + 10;
struct Edge
{
    int _from, _to, _cap, _flow;
    Edge(int f, int t, int c, int l):_from(f), _to(t), _cap(c), _flow(l)
    {};
};

vector<int> G[MAXN];
vector<Edge> edges;
int a[MAXN], p[MAXN];
int n, m;

int Get_Flow()
{
    LL flow = 0;
    while (1)
    {
        memset(a, 0, sizeof(a));
        queue<int> que;
        que.push(1);
        a[1] = INF;
        while (!que.empty())
        {
            int x = que.front();
            que.pop();
            for (int i = 0;i < G[x].size();i++)
            {
                Edge & e = edges[G[x][i]];
                if (!a[e._to] && e._cap > e._flow)
                {
                    p[e._to] = G[x][i];
                    a[e._to] = min(a[x], e._cap - e._flow);
                    que.push(e._to);
                }
            }
            if (a[m])
                break;
        }
        if (a[m] == 0)
            break;
        for (int u = m;u != 1;u = edges[p[u]]._from)
        {
            edges[p[u]]._flow += a[m];
            edges[p[u] ^ 1]._flow -= a[m];
        }
        flow += a[m];
    }
    return flow;
}

void Init()
{
    edges.clear();
    for (int i = 1;i <= m;i++)
        G[i].clear();
}

int main()
{
    int l, r, c;
    while(~scanf("%d%d", &n, &m))
    {
        Init();
        for (int i = 1; i <= n; i++)
        {
            scanf("%d%d%d", &l, &r, &c);
            edges.emplace_back(l, r, c, 0);
            edges.emplace_back(r, l, 0, 0);
            int sum = edges.size();
            G[l].push_back(sum - 2);
            G[r].push_back(sum - 1);
        }
        printf("%d
", Get_Flow());
    }

    return 0;
}