C++图论之Bellman-Ford算法和SPFA算法的实现

#include <iostream>
#include <cstring>
using namespace std;
 
const int N = 510, M = 10010;
 
struct Edge
{
    int a, b, w;
}e[M]; // 存下每一条即可
int dist[N];
int back[N]; // 备份数组放置串联
int n, m, k;
 
void bellman_ford()
{
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    
    for(int i = 0; i < k; i ++ ) // 不超过k条边
    {
        memcpy(back, dist, sizeof back);
        for(int j = 0; j < m; j ++ ) // 遍历所有边
        {
            int a = e[j].a, b = e[j].b, w = e[j].w;
            dist[b] = min(dist[b], back[a] + w);
        }
    }
}
 
int main()
{
    cin >> n >> m >> k;
    for(int i = 0; i < m; i ++ )
    {
        int a, b, w;
        scanf("%d%d%d", &a, &b, &w);
        e[i] = {a, b, w};
    }
    
    bellman_ford();
    if(dist[n] > 0x3f3f3f3f / 2) puts("impossible");
    else cout << dist[n] << endl;
    
    return 0;
}

#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>
 
using namespace std;
 
const int N = 1e6 + 10;
 
int n, m;
int h[N], e[N], w[N], ne[N], idx;
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()
{
    memset(dist, 0x3f, sizeof dist);
    queue<int> q;
    dist[1] = 0;
    st[1] = true;
    q.push(1);
    
    while(q.size())
    {
        int t = q.front();
        q.pop();
        
        st[t] = false;
        
        for(int i = h[t]; ~i; 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] = true;
                }
            }
        }
    }
}
 
int main()
{
    scanf("%d%d", &n, &m);
 
    memset(h, -1, sizeof h);
    while (m -- )
    {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
    }
 
    spfa();
    
    if(dist[n] == 0x3f3f3f3f) puts("impossible");
    else printf("%d",dist[n]);
 
    return 0;
}