poj3159

spfa算法可以用栈代替队列，入栈时注意判断是否已在栈中。

开始以为是要找最长路，其实是要找最短路。

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define maxn 30005
#define maxm 150005

struct Edge
{
    int v, w, next;
}edge[maxm];

int n, m, head[maxn], mystack[maxn], edgenum, top, mytime[maxn];
bool instack[maxn];

void init()
{
    edgenum = 0;
    memset(head, -1, sizeof(head));
    for (int i = 0; i < m; i++)
    {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        a--;
        b--;
        edge[edgenum].next = head[a];
        edge[edgenum].v = b;
        edge[edgenum].w = c;
        head[a] = edgenum;
        edgenum++;
    }
}

void work()
{
    memset(instack, 0, sizeof(instack));
    memset(mytime, -1, sizeof(mytime));
    instack[0] = 1;
    mystack[0] = 0;
    mytime[0] = 0;
    top = 1;
    while (top)
    {
        int temp = mystack[--top];
        instack[temp] = false;
        for (int i = head[temp]; i != -1; i = edge[i].next)
        {
            if (mytime[edge[i].v] == -1 || mytime[edge[i].v] > mytime[temp] + edge[i].w)
            {
                mytime[edge[i].v] = mytime[temp] + edge[i].w;
                if (!instack[edge[i].v])
                {
                    mystack[top++] = edge[i].v;
                    instack[edge[i].v] = true;
                }
            }
        }
    }
    printf("%d\n", mytime[n - 1]);
}

int main()
{
    //freopen("D:\\t.txt", "r", stdin);
    scanf("%d%d", &n, &m);
    init();
    work();
}