【模板】逆单源最短（反向建图） + spfa

题目要求：不仅要求单源最短路径，还要求其余点到该点的最短路径

做法：建立反图求逆单源最短路径，至于单源最短路径选择合适于题目即可

参考题目

#include <iostream>
#include <queue>
#include <cstring>

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;
typedef pair<int, PII> PIP;

const int N = 1000010;

int h[N], e[N], ne[N], idx;
int w[N], dist[N];
bool st[N];
PIP tmp[N];

int n, m;

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

void spfa()
{
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    
    queue<int> q;
    st[1] = true;
    q.push(1);
    
    while (q.size())
    {
        int t = q.front();
        q.pop();
        st[t] = false;
        
        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] = true;
                    q.push(j);
                }
            }
        }
    }
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    
    cin >> n >> m;
    
    for(int i = 0; i < m; i ++ )
    {
        int a, b, c;
        cin >> a >> b >> c;
        tmp[i] = {a, {b, c}};
    }
    
    memset(h, -1, sizeof h);
    for (int i = 0; i < m; i ++ ) add(tmp[i].first, tmp[i].second.first, tmp[i].second.second);
    
    spfa();
    
    LL sum = 0;
    for (int i = 1; i <= n; i ++ ) sum += dist[i];
    
    memset(h, -1, sizeof h);
    memset(st, 0, sizeof st);
    idx = 0;
    
    for (int i = 0; i < m; i ++) add(tmp[i].second.first, tmp[i].first, tmp[i].second.second);
    
    spfa();
    
    for (int i = 1; i <= n; i ++ ) sum += dist[i];
    
    cout << sum;
}