朴素
O(nm)

int k;//边数限制
int dist[N], back[N];

struct Edge
{
    int a, b, w;
} e[M];

void bellman_ford(int start)
{
    memset(dist, 0x3f, sizeof(dist));
    dist[start] = 0;
    for (int i = 0; i < k; i++)
    {
        memcpy(back, dist, sizeof(back));//无边数限制可删除
        for (int j = 0; j < m; j++)
        {
            int a = e[j].a, b = e[j].b, w = e[j].w;
            dist[b] = min(dist[b], back[a] + w);//无边数限制:dist[b] = min(dist[b], dist[a] + w);
        }
    }
}

队列优化版
O(m),可能被卡到O(nm)

int h[N], w[M], e[M], ne[M], idx;//memset(h, -1, sizeof h);
int dist[N];
bool st[N];

void add(int a, int b, int c)
{
    e[idx] = b;
    ne[idx] = h[a];
    w[idx] = c;
    h[a] = idx++;
}

void spfa(int start)
{
    queue<int>q;
    memset(dist,0x3f,sizeof dist);
    dist[start]=0;
    q.push(start);
    st[start]=1;
    while(q.size())
    {
        int t=q.front();
        q.pop();
        st[t]=0;
        for(int i=h[t];i!=-1;i=ne[i])
        {
            int j=e[i];
            if(dist[j]>dist[t]+w[i])
            {
                dist[j]=dist[t]+w[i];
                if(!st[j])
                {
                    q.push(j);
                    st[j]=1;
                }
            }
        }
    }
}

spfa(SLF优化)

int h[N], w[M], e[M], ne[M], idx; //memset(h, -1, sizeof h);
int dist[N];
bool st[N];

void add(int a, int b, int c)
{
    e[idx] = b;
    ne[idx] = h[a];
    w[idx] = c;
    h[a] = idx++;
}

void spfa(int start)
{
    deque<int> q;
    memset(dist, 0x3f, sizeof dist);
    dist[start] = 0;
    q.push_back(start);
    st[start] = 1;
    while (q.size())
    {
        int t = q.front();
        q.pop_front();
        st[t] = 0;
        for (int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            if (dist[j] > dist[t] + w[i])
            {
                dist[j] = dist[t] + w[i];
                if (!st[j])
                {
                    st[j] = 1;
                    if (q.size() && dist[j] < dist[q.front()])
                    {
                        q.push_front(j);
                    }
                    else
                    {
                        q.push_back(j);
                    }
                }
            }
        }
    }
}