poj3204Ikki's Story I - Road Reconstruction

这题只是最大流的一个模板题目。 

题目要求怎样修改道路才会增加最大流。 

思路就是在残余的网络流里面在某一个零边增加一个单位的网络流。

查看是否有到首都的道路。

#include<stdio.h>
#include<string.h>
#include<math.h>
#define inf 0x7ffffff
#define maxn 505

int map[maxn][maxn], visit[maxn], e[maxn][maxn];
int f[maxn][maxn] , pre[maxn], prev[maxn];
int queue[maxn * 2];
int n, m;

inline int min(int a, int b)
{
    return a > b? b:a;
}

void init()
{
    memset(map, 0, sizeof(map));
    memset(e, 0, sizeof(e));
    memset(f, 0, sizeof(f));
    int x, y, val;
    for (int i = 1; i <= m;i ++)
    {
        scanf("%d%d%d", &x, &y, &val);
        x ++; y ++;
        map[x][0] ++;
        map[x][map[x][0]] = y;
        e[x][ y] = val;
    }
    return;
}

bool flow()
{
    int front = 1, rear = 0;
    pre[1] = 1;
    prev[1] = inf;
    queue[front] = 1;
    memset(visit, 0, sizeof(visit));
    visit[1] = 1;

    while (front != rear)
    {
        rear ++;
        int x = queue[rear];
        for (int i = 1; i <= map[x][0]; i ++)
        {
            int start = map[x][i];
            if (visit[start] == 0)
            {
                if (e[x][start] > f[x][start])
                {
                    pre[start] = x;
                    prev[start] = min(e[x][start] - f[x][start], prev[x]);
                    visit[start] = 1;
                    front ++;
                    queue[front] = start;
                    if (start == n)
                    {
                        return true;
                    }
                }
                else if (f[start][x] > 0)
                {
                    pre[start] = -x;
                    prev[start] = min(prev[x], f[start][x]);
                    visit[start] = 1;
                    front ++;
                    queue[front] = start;
                    if (start == n)
                    {
                        return true;
                    }
                }
            }
        }
    }
    return false;
}

void update()
{
    int x = n;
    while (x != 1)
    {
        int y = x;
        x = pre[x];
        if (x < 0)
        {
            x = -x;
            f[y][x] -= prev[n];
        }
        else
        {
            f[x][y] += prev[n];
        }
    }
    return;
}

void funs()
{
    int num  = 0;
    while (flow()) update();
    for (int i = 1; i < n; i ++)
    {
        for (int j = 1; j <= map[i][0]; j ++)
        {
            int x = map[i][j];
            if (e[i][x] == f[i][x])
            {
                e[i][x] ++;
                if (flow()) num ++;
                e[i][x] --;
            }
        }
    }
    printf("%d\n", num);
    return;
}

int main()
{
    while (scanf("%d%d", &n, &m)  != EOF)
    {
        init();
        funs();
    }
    return 0;
}