【算法】图论 —— Dijkstra算法 python

引入

求非负权边的单源最短路

时间复杂度 O($mlogn$)

模板

https://www.luogu.com.cn/problem/P4779

import heapq as hq  
  
def dijkstra(s):  
    # dis表示从s到i的最短路  
    dis = [float('inf')] * (n + 1)  
    # vis表示i是否出队列  
    vis = [0] * (n + 1)  
    q = []  
    dis[s] = 0  
    hq.heappush(q, (0, s))  
    while q:  
        d, u = hq.heappop(q)  
        if vis[u]:  
            continue  
        vis[u] = 1  
        for v, w in g[u]:  
            dis[v] = min(dis[v], dis[u] + w)  
            hq.heappush(q, (dis[v], v))  
    for i in range(1, n + 1):  
        if dis[i] == float('inf'):  
            dis[i] = -1  
    return dis[1:]  
  
n, m, s= map(int, input().split())  
g = [[] for _ in range(n + 1)]  
for _ in range(m):  
    u, v, w = map(int, input().split())  
    g[u].append((v, w))  
print(*dijkstra(s))

实战演练

L2 001 紧急救援

维护多个数组并得到具体路径

import heapq as hq  
import sys  
input = sys.stdin.readline  
inf = float('inf')  
  
def dijkstra(s, ed):  
    q = []  
    vis = [0] * n  
    dis = [inf] * n  
    pre = [-1] * n  
    cnt = [0] * n  # 最短路径数量  
    res = [0] * n  # 能召集的最大救援队数量  
  
    dis[s] = 0  
    cnt[s] = 1  
    res[s] = a[s]  
    hq.heappush(q, (0, s))  
  
    while q:  
        d, u = hq.heappop(q)  
        if u == ed:  
            return dis[ed], cnt[ed], res[ed], pre  
        if vis[u]:  
            continue  
        vis[u] = 1  
        for (v, w) in g[u]:  
            if dis[v] > dis[u] + w:  
                dis[v] = dis[u] + w  
                cnt[v] = cnt[u]  
                res[v] = res[u] + a[v]  
                pre[v] = u  
                hq.heappush(q, (dis[v], v))  
            elif dis[v] == dis[u] + w:  
                cnt[v] += cnt[u]  # 更新最短路径数量  
                if res[v] < res[u] + a[v]:  
                    res[v] = res[u] + a[v]  
                    pre[v] = u  
    return -1, -1, -1, pre  
  
n, m, s, ed = map(int, input().split())  
a = list(map(int, input().split()))  
g = [[] for _ in range(n)]  
for _ in range(m):  
    u, v, w = map(int, input().split())  
    g[u].append((v, w))  
    g[v].append((u, w))  # 无向图  
  
dist, cnt, res, pre = dijkstra(s, ed)  
if dist != -1:  
    print(cnt, res)  
    path = []  
    while ed != -1:  # 从终点回溯路径  
        path.append(ed)  
        ed = pre[ed]  
    path.reverse()  # 反转路径  
    print(' '.join(map(str, path)))

https://www.lanqiao.cn/problems/3818/learning/

import sys  
import heapq as hq  
input = sys.stdin.readline  
  
def get(c):  
    if c == '.':  
        return 0  
    else:  
        return ord(c) - ord('A') + 1  
  
def dijkstra():  
    q = []  
    dis = [[float('inf')] * m for _ in range(n)]  
    vis = [[0] * m for _ in range(n)]  
    dis[x1][y1] = 0  
    hq.heappush(q, (0, x1, y1))  
    while q:  
        d, x, y = hq.heappop(q)  
        if x == x2 and y == y2:  
            return d  
        if vis[x][y]:  
            continue  
        vis[x][y] = 1  
        for dx, dy in [(1, 0), (-1, 0), (0, 1), (0, -1)]:  
            nx, ny = x + dx, y + dy  
            if 0 <= nx < n and 0 <= ny < m and a[nx][ny] != '#' and not vis[nx][ny]:  
                dis[nx][ny] = min(dis[nx][ny], dis[x][y] + get(a[nx][ny]))  
                hq.heappush(q, (dis[nx][ny], nx, ny))  
    return float('inf')  
  
n, m = map(int, input().split())  
x1, y1, x2, y2 = map(int, input().split())  
x1 -= 1  
x2 -= 1  
y1 -= 1  
y2 -= 1  
a = [input() for _ in range(n)]  
e = int(input())  
  
if e >= dijkstra():  
    print('Yes')  
else:  
    print('No')

https://ac.nowcoder.com/acm/contest/99909/E

对于每条边 $(u,v)$，如果这条边在某条最短路上的话，则一定满足:
dis[u] + w + dis[v] == dis[n] ,即$最短路从1到u，再从u到v，再从v到n$。(或者是$u,v$反过来)

import sys  
input = sys.stdin.readline  
inf = float('inf')  
import heapq as hp  
  
def dijkstra(s):  
    dis = [inf] * (n + 1)  
    vis = [0] * (n + 1)  
    q = [(0, s)]  
    dis[s] = 0  
    while q:  
        d, u = hp.heappop(q)  
        if vis[u]:  
            continue  
        vis[u] = 1  
        for v, w in g[u]:  
            if dis[v] > dis[u] + w:  
                dis[v] = dis[u] + w  
                hp.heappush(q, (dis[v], v))  
    return dis  
  
t = int(input())  
for _ in range(t):  
    n, m = map(int, input().split())  
    g = [[] for _ in range(n + 1)]  
    edge = []  
    for i in range(m):  
        u, v, w = map(int, input().split())  
        g[u].append((v, w))  
        g[v].append((u, w))  
        edge.append((u, v, w))  
  
    dis1 = dijkstra(1)  
    disn = dijkstra(n)  
    res = []  
    for u, v, w in edge:  
        # 检查是否在最短路径中  
        if dis1[u] != inf and disn[v] != inf and (dis1[u] + w + disn[v] == dis1[n]):  
            res.append('1')  
        elif dis1[v] != inf and disn[u] != inf and (dis1[v] + w + disn[u] == dis1[n]):  
            res.append('1')  
        else:  
            res.append('0')  
    print(''.join(res))

END
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