acwing 1129

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题意：无向边的最短距离

考虑边数小于点数，用SPFA算法，算法复杂度O(me)​

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

using namespace std;

const int N = 2600, M = 6300 * 2;

typedef pair<int, int> PII;

int T;
int e[M], w[M], ne[M], h[N], idx;
int dist[N];
int q[N];
bool st[N];

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

int spfa(int s, int ed) {
    memset(dist, 0x3f, sizeof dist);
    dist[s] = 0;
    st[s] = true;
    int hh = 0, tt = 1;
    q[0] = s;

    while(hh != tt) {
        int t = q[hh ++];
        if (hh == N) hh = 0;
        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[++ tt] = j;
                    if (tt == N) tt = 0;
                }
            }
        }
    }
    return dist[ed];
}

int main() {
    memset(h, -1, sizeof h);
    int c, s, e;
    cin >> T >> c >> s >> e;
    for (int i = 0; i < c; i ++) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
        add(b, a, c);
    }
    cout << spfa(s, e) << endl;
    return 0;
}