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851. spfa求最短路
https://www.acwing.com/problem/content/submission/code_detail/18520113/
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#include <bits/stdc++.h>
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#include <iostream>
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#include <algorithm>
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#include <queue>
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#include <utility>
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#include <vector>
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using namespace std;
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using std::cin;
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using std::cout;
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const char endl = '\n';
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int n, m, u, v, w, dist[100005];
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vector<pair<int, int>> g[100005];
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const int N = 1e5 + 5;
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const int INF = 0x3f3f3f3f;
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int n, m, dist[N];
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std::vector<std::pair<int, int>> g[N];
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bool vis[N];
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void spfa() {
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memset(dist, 0x3f, sizeof(dist));
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std::fill_n(dist, N, INF);
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std::queue<int> q;
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q.emplace(1);
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dist[1] = 0;
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queue<int> q;
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q.push(1);
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vis[1] = true;
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while (!q.empty()) {
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int t = q.front();
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int u = q.front();
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q.pop();
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for (auto i : g[t]) {
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if (dist[i.first] > dist[t] + i.second) {
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dist[i.first] = dist[t] + i.second;
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q.push(i.first);
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vis[u] = false;
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for (auto e : g[u]) {
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int v = e.first,
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w = e.second;
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if (dist[v] > dist[u] + w) {
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dist[v] = dist[u] + w;
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if (!vis[v]) {
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vis[v] = true;
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q.emplace(v);
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}
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}
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}
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}
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}
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int main() {
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std::ios::sync_with_stdio(false);
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cin.tie(nullptr);
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cin >> n >> m;
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for (int i = 0; i < m; i++) {
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for (int i = 1, u, v, w; i <= m; i++) {
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cin >> u >> v >> w;
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g[u].push_back(make_pair(v, w));
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g[u].emplace_back(v, w);
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}
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spfa();
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if (dist[n] == 0x3f3f3f3f) {
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if (dist[n] == INF) {
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cout << "impossible" << endl;
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} else {
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cout << dist[n] << endl;
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}
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return 0;
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}
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