题目

代码1（spfa）

#include <bits/stdc++.h>
using namespace std;
#define int long long
int n, m, s, t, cnt;

struct edge {
	int to, cost, nex;
} e[400020];
int dis[200020];
int head[200020];
bool vis[200020];

void add(int x, int y, int z) {
	e[++cnt].to = y;
	e[cnt].cost = z;
	e[cnt].nex = head[x];
	head[x] = cnt;
}

void spfa() {
	queue<int>Q;
	for (int i = 1; i <= n; i++)
		dis[i] = 1e15;
	dis[1] = 0;
	vis[1] = 1;
	Q.push(1);
	while (!Q.empty()) {
		int u = Q.front();
		Q.pop();
		vis[u] = 0;
		for (int i = head[u]; i; i = e[i].nex) {
			int v = e[i].to;
			if (dis[v] > dis[u] + e[i].cost) {
				dis[v] = dis[u] + e[i].cost;
				if (!vis[v]) {
					vis[v] = 1;
					Q.push(v);
				}
			}
		}
	}
}

signed main() {
	cin >> n >> m >> s >> t;
	for (int i = 1; i <= m; i++) {
		int x, y;
		cin >> x >> y;
		add(x, y, 1);
		add(y, x, 1);
	}
	spfa();
	int ans = -1;
	for (int i = 1; i <= n; i++) {
		ans = max(ans, dis[i]);
	}
	cout << ans + t + 1;
}

TLE了四个点

代码2（dijkastra）

#include <bits/stdc++.h>
using namespace std;
#define int long long
int n, m, s, t, cnt;

struct edge {
	int to, cost, nex;
} e[300020];
int head[300020];
int dis[300020];
bool vis[300020];

inline void add(int x, int y, int z) {
	e[++cnt].to = y;
	e[cnt].cost = z;
	e[cnt].nex = head[x];
	head[x] = cnt;
}

void dijkstra() {
	memset(dis, 127, sizeof(dis));
	dis[s] = 0;
	int q = n, st = s;
	while (q--) {
		vis[st] = 1;
		for (int i = head[st]; i != 0; i = e[i].nex) {
			if (!vis[e[i].to] && dis[st] + e[i].cost < dis[e[i].to]) {
				dis[e[i].to] = dis[st] + e[i].cost;
			}
		}
		int minn = 9e18;
		for (int i = 1; i <= n; i++) {
			if (!vis[i] && dis[i] < minn) {
				minn = dis[i];
				st = i;
			}
		}
	}
}

signed main() {
	cin >> n >> m >> s >> t;
	for (int i = 1; i <= m; i++) {
		int x, y;
		cin >> x >> y;
		add(x, y, 1);
		add(y, x, 1);
	}
	dijkstra();
	int ans = -1;
	for (int i = 1; i <= n; i++) {
		ans = max(ans, dis[i]);
	}
	cout << ans + t + 1;
}

WA了四个点