链式前向星

前向星是一种特殊的边集数组,我们把边集数组中的每一条边按照起点从小到大排序,如果起点相同就按照终点从小到大排序,

但是利用前向星会有排序操作,如果用快排时间至少为O(nlog(n))

如果用链式前向星,加入next索引模拟指针指向下一个点的位置，就可以避免排序.

#include <bits/stdc++.h>
using namespace std;
#define de(a) cout << #a << " = " << a << endl
#define rep(i, a, n) for (int i = a; i < n; i++)
#define per(i, a, n) for (int i = n - 1; i >= a; i--)
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> PII;
typedef pair<double, double> PDD;
typedef vector<int, int> VII;
#define inf 0x3f3f3f3f
const ll INF = 0x3f3f3f3f3f3f3f3f;
const ll MAXN = 1e3 + 7;
const ll MAXM = 1e5 + 7;
const ll MOD = 1e9 + 7;
const double eps = 1e-6;
const double pi = acos(-1.0);
int n, m;
struct Edge
{
    int from, to, val;
    int next;
} E[MAXM];
int cnt = -1;
int head[MAXN];
void add(int from, int to, int val)
{
    E[++cnt].from = from;
    E[cnt].to = to;
    E[cnt].val = val;
    E[cnt].next = head[from];
    head[from] = cnt;
}
int main()
{
    memset(head, -1, sizeof(head));
    cin >> n >> m;
    for (int i = 0; i < m; i++)
    {
        int u, v, val;
        cin >> u >> v >> val;
        add(u, v, val); //建边
    }
    int x = 1; //遍历从结点1出发的所有有向边
    for (int i = head[x]; i != -1; i = E[i].next)
        printf("from=%d to=%d v=%d\n", E[i].from, E[i].to, E[i].val);
    return 0;
}