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OI-codes/Luogu/P2169/P2169.cpp
2022-01-01 20:40:59 +08:00

93 lines
1.8 KiB
C++

#include <iostream>
#include <limits>
#include <queue>
#include <stack>
#include <vector>
using std::cin;
using std::cout;
using std::endl;
// Limits
const int N = 200005;
// Graphs
int w[N];
std::vector<std::pair<int, int>> g[N];
// Tarjan
int cnt, dfn[N], low[N];
int scc_cnt, id[N];
std::stack<int> st;
bool vis[N];
void tarjan(int u) {
dfn[u] = low[u] = ++cnt;
st.push(u);
vis[u] = true;
for (auto v : g[u]) {
if (!dfn[v.first]) {
tarjan(v.first);
low[u] = std::min(low[u], low[v.first]);
} else if (vis[v.first]) {
low[u] = std::min(low[u], dfn[v.first]);
}
}
if (low[u] == dfn[u]) {
scc_cnt++;
int v;
do {
v = st.top();
st.pop();
vis[v] = false;
id[v] = scc_cnt;
} while (v != u);
}
}
// Shortest Path
int dist[N];
void spfa() {
std::fill(dist, dist + N, 0x3f3f3f3f);
std::fill(vis, vis + N, false);
std::queue<int> q;
q.push(1);
dist[1] = 0;
vis[1] = true;
while (!q.empty()) {
auto u = q.front();
q.pop();
vis[u] = 0;
for (auto v : g[u]) {
int w = v.second;
if (id[u] == id[v.first]) w = 0;
if (dist[v.first] > dist[u] + w) {
dist[v.first] = dist[u] + w;
if (!vis[v.first]) {
q.push(v.first);
vis[v.first] = true;
}
}
}
}
}
int main() {
int n, m;
cin >> n >> m;
for (int i = 1; i <= m; i++) {
int u, v, w;
cin >> u >> v >> w;
g[u].push_back(std::make_pair(v, w));
}
for (int i = 1; i <= n; i++) {
if (!dfn[i]) {
tarjan(i);
}
}
spfa();
cout << dist[n] << endl;
return 0;
}