hdu 2485 Destroying the bus stations 搜索(迭代加深)

hdu 2485 Destroying the bus stations 搜索(迭代加深)

//hdu 2485 Destroying the bus stations

//搜索(迭代加深)

//没有反向边



#include <stdio.h>

#include <string.h>





#define N 55

#define INF 1<<30

#define eps 1e-5



int n_node, n_edge, limit, len;//len 记录广搜最短路的长度

int limit_depth;    //记录当前神搜限制的深度

bool success;       //标记是否找到答案





int que[N], road[N][N]; //que广搜时用的队列,road(i, j)表示深度为i时的广搜的最短路

int pre[N]; //pre记录广搜路径

int end[4005], head[N], next[4005], tot;  //邻接表

bool del[N];    //记录被删的点



void add_edge(int from, int to)     //add edge into adjacency list

{

    end[++tot] = to;

    next[tot] = head[from];

    head[from] = tot;

}



bool bfs()   //breadth first search

{

    for(int i = 1; i <= n_node; ++i)

        pre[i] = 0;

    int top = 0, tail = 0;

    que[++top] = 1;

    while(tail < top)

    {

        int now = que[++tail];

        for(int i = head[now]; i != -1; i = next[i])

        {

            int point = end[i];

            if(del[point] == false && pre[point] == 0)

            {

                pre[point] = now;       //记录广搜路径

                que[++top] = point;         //入队

                if(point == n_node)

                    return true;

            }

        }

    }

    return false;

}



void dfs(int dep)   //depth first search

{

    if(success == true)

        return;

    bool find = bfs();   //广搜找最短



    if(find == false)      //找不到最短路了

    {

        success = true;

        return;

    }

    int cnt = 0;    //记录最短路的长度

    //que[top]为最后一个节点,i == 1时就跳出循环，所以没有包括起点和终点

    for(int i = pre[n_node]; i != 1; i = pre[i])

        road[dep][cnt++] = i;

    if(cnt >= limit)

    {

        success = true;

        return;

    }



    if(dep > limit_depth)   //深度大于限制的深度，也就是不能再删点了

        return;



    //不可以删最后一个(i=cnt-2的点)和第一个节点(i=0的点)

    for(int i = cnt - 1; i >= 0; --i)

    {

        int del_point = road[dep][i];

        del[del_point] = true;

        dfs(dep + 1);

        del[del_point] = false;

    }

}



int main()

{

    //freopen("in.txt", "r", stdin);

    bool have[N][N];

    while(scanf("%d%d%d", &n_node, &n_edge, &limit), n_node || n_edge || limit)

    {

        tot = 0;    //邻接表下标

        memset(have, false, sizeof(have));



        for(int i = 0; i <= n_node; ++i)    //初始化

        {

            head[i] = -1;

            del[i] = false;

        }



        for(int i = 0; i < n_edge; ++i)

        {

            int from, to;

            scanf("%d%d", &from, &to);

            if(have[from][to] == false)

            {

                have[from][to] = true;

                add_edge(from, to);

            }

        }



        int m = n_node - 2;

        success = false;

        //迭代加深搜索，限制深度为i，即限制最多删i个点

        for(int i = 0; i <= m; ++i)

        {

            limit_depth = i;

            dfs(1);

            if(success == true)

                break;

        }

        printf("%d\n", limit_depth);

    }

    return 0;

}