POJ 1324 Pots(BFS)

题目链接：POJ 1324 Pots

思路挺清晰的，就是写起来好麻烦，主要难点就是打印路径吧，可以在结构体里加一个pre表示前一个状态在vector中的下标，初始状态的pre是-1。

#include <iostream>
#include <cstring>
#include <cstdio>
#include <queue>
#include <vector>
#include <stack>

using namespace std;

const int MAX_N = 100 + 10;
const int INF = (1 << 29);

int vis[MAX_N][MAX_N], arr[3];
int res;
struct State
{
    int A, B, op, dis, pre, from, to, now;
    State(int A = 0, int B = 0, int now = 0, int op = 0, int dis = 0, int pre = -1,  int from = 0, int to = 0) : A(A), B(B), now(now), op(op), dis(dis), pre(pre), from(from), to(to) {};
};

queue <State> Q;
vector <State> V;
stack <int> S;

void BFS()
{
    State a;
    int dA, dB, pre;
    while(!Q.empty())
    {
        a = Q.front();
        Q.pop();
        pre = a.now;
        // fill 1;
        dA = arr[0], dB = a.B;
        if(!vis[dA][dB])
        {
            V.push_back(State(dA, dB, 0, 0, a.dis + 1, pre, 1));
            Q.push(State(dA, dB, V.size() - 1, 0, a.dis + 1, pre));
            vis[dA][dB] = 1;
            if(dA == arr[2] || dB == arr[2])
            {
                res = a.dis + 1;
                return ;
            }
        }
        // fill 2;
        dA = a.A, dB = arr[1];
        if(!vis[dA][dB])
        {
            V.push_back(State(dA, dB, 0, 0, a.dis + 1, pre, 2));
            Q.push(State(dA, dB, V.size() - 1, 0, a.dis + 1, pre));
            vis[dA][dB] = 1;
            if(dA == arr[2] || dB == arr[2])
            {
                res = a.dis + 1;
                return ;
            }
        }
        // pour 1 -> 2
        if(a.A + a.B <= arr[1])
        {
            dA = 0, dB = a.A + a.B;
            if(!vis[dA][dB])
            {
                V.push_back(State(dA, dB, 0 , 1, a.dis + 1, pre, 1, 2));
                Q.push(State(dA, dB, V.size() - 1, 1, a.dis + 1, pre));
                vis[dA][dB] = 1;
                if(dA == arr[2] || dB == arr[2])
                {
                    res = a.dis + 1;
                    return ;
                }
            }

        }
        else
        {
            dA = a.A - (arr[1] - a.B), dB = arr[1];
            if(!vis[dA][dB])
            {
                V.push_back(State(dA, dB, 0 , 1, a.dis + 1, pre, 1, 2));
                Q.push(State(dA, dB, V.size() - 1, 1, a.dis + 1, pre));
                vis[dA][dB] = 1;
                if(dA == arr[2] || dB == arr[2])
                {
                    res = a.dis + 1;
                    return ;
                }
            }

        }
        // pour 2 -> 1
        if(a.A + a.B <= arr[0])
        {
            dA = a.A + a.B, dB = 0;
            if(!vis[dA][dB])
            {
                V.push_back(State(dA, dB, 0 , 1, a.dis + 1, pre, 2, 1));
                Q.push(State(dA, dB, V.size() - 1, 1, a.dis + 1, pre));
                vis[dA][dB] = 1;
                if(dA == arr[2] || dB == arr[2])
                {
                    res = a.dis + 1;
                    return ;
                }
            }
        }
        else
        {
            dA = arr[0], dB = a.B - (arr[0] - a.A);
            if(!vis[dA][dB])
            {
                V.push_back(State(dA, dB, 0 , 1, a.dis + 1, pre, 2, 1));
                Q.push(State(dA, dB, V.size() - 1, 1, a.dis + 1, pre));
                vis[dA][dB] = 1;
                if(dA == arr[2] || dB == arr[2])
                {
                    res = a.dis + 1;
                    return ;
                }
            }
        }
        // drop 1
        dA = 0, dB = a.B;
        if(!vis[dA][dB])
        {
            V.push_back(State(dA, dB, 0 , 2, a.dis + 1, pre, 1));
            Q.push(State(dA, dB, V.size() - 1, 2, a.dis + 1, pre));
            vis[dA][dB] = 1;
            if(dA == arr[2] || dB == arr[2])
            {
                res = a.dis + 1;
                return ;
            }
        }
        // drop 2
        dA = a.A, dB = 0;
        if(!vis[dA][dB])
        {
            V.push_back(State(dA, dB, 0 , 2, a.dis + 1, pre, 2));
            Q.push(State(dA, dB, V.size() - 1, 2, a.dis + 1, pre));
            vis[dA][dB] = 1;
            if(dA == arr[2] || dB == arr[2])
            {
                res = a.dis + 1;
                return ;
            }
        }
    }
}
void print()
{
    printf("%d\n", res);
    int i = V.size() - 1;
    while(V[i].pre != -1)
    {
        S.push(i);
        i = V[i].pre;
    }
    while(!S.empty())
    {
        if(V[S.top()].op == 0)
            printf("FILL(%d)\n", S.top(), V[S.top()].from);
        else if(V[S.top()].op == 1)
            printf("POUR(%d,%d)\n", V[S.top()].from, V[S.top()].to);
        else if(V[S.top()].op == 2)
            printf("DROP(%d)\n", V[S.top()].from);
        S.pop();
    }
}
int main()
{
    //freopen("in.txt", "r", stdin);
    res = INF;
    memset(vis, 0, sizeof(vis));
    scanf("%d%d%d", &arr[0], &arr[1], &arr[2]);
    vis[0][0] = 1;
    V.push_back(State(0, 0));
    Q.push(State(0, 0, V.size() - 1));
    BFS();
    if(res == INF)
        printf("impossible\n");
    else
        print();
    return 0;
}