【最短路】SSL_1297 GF打Dota

题意

给出一张图，求出从点$1$到点$n$的次短路径。

思路

分别以起点和终点跑一次最短路，然后枚举一条边来绕长最短路变成次短路，更新答案。

代码

#include<queue>
#include<cstdio>
#include<cstring>
#include<algorithm>

int n, m, tot = -1, p, ans;
int ver[100001], next[100001], head[100001], edge[100001];
int dis[10001], dise[10001], v[10001];

void add(int u, int v, int w) {
	ver[++tot] = v;
	next[tot] = head[u];
	edge[tot] = w;
	head[u] = tot;
}

void spfa() {
	memset(dis, 127 / 3, sizeof(dis));
	std::queue<int> q;
	q.push(1);
	v[1] = 1;
	dis[1] = 0;
	while (q.size()) {
		int x = q.front();
		q.pop();
		v[x] = 0;
		for (int i = head[x]; ~i; i = next[i]) {
			int y = ver[i];
			if (dis[x] + edge[i] < dis[y]) {
				dis[y] = dis[x] + edge[i];
				if (!v[y]) {
					v[y] = 1;
					q.push(y);
				}
			}
		}
	}
}

void spfaa() {
	memset(v, 0, sizeof(v));
	memset(dise, 127 / 3, sizeof(dise));
	std::queue<int> q;
	q.push(n);
	v[n] = 1;
	dise[n] = 0;
	while (q.size()) {
		int x = q.front();
		q.pop();
		v[x] = 0;
		for (int i = head[x]; ~i; i = next[i]) {
			int y = ver[i];
			if (dise[x] + edge[i] < dise[y]) {
				dise[y] = dise[x] + edge[i];
				if (!v[y]) {
					v[y] = 1;
					q.push(y);
				}
			}
		}
	}
}

int main() {
	memset(head, -1, sizeof(head));
	scanf("%d %d", &n, &m);
	for (int i = 1; i <= m; i++) {
		int x, y, z;
		scanf("%d %d %d", &x, &y, &z);
		add(x, y, z);
		add(y, x, z);
	}
	scanf("%d", &p);
	spfa();
	if (!p) printf("%d", dis[n]);
	else {
		ans = 2147483647;
		spfaa();
		for (int i = 0; i <= tot; i += 2) {
			int x = ver[i ^ 1];
			int y = ver[i];
			int s = dis[x] + dise[y] + edge[i];
			if (s > dis[n]) ans = std::min(ans, s);
			s = dis[y] + dise[x] + edge[i];
			if (s > dis[n]) ans = std::min(ans, s);
		}
		printf("%d", ans);
	}
}