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困难 |
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给你一个数组 routes ，表示一系列公交线路，其中每个 routes[i] 表示一条公交线路，第 i 辆公交车将会在上面循环行驶。
- 例如，路线
routes[0] = [1, 5, 7]表示第0辆公交车会一直按序列1 -> 5 -> 7 -> 1 -> 5 -> 7 -> 1 -> ...这样的车站路线行驶。
现在从 source 车站出发（初始时不在公交车上），要前往 target 车站。 期间仅可乘坐公交车。
求出 最少乘坐的公交车数量 。如果不可能到达终点车站，返回 -1 。
示例 1：
输入：routes = [[1,2,7],[3,6,7]], source = 1, target = 6 输出：2 解释：最优策略是先乘坐第一辆公交车到达车站 7 , 然后换乘第二辆公交车到车站 6 。
示例 2：
输入：routes = [[7,12],[4,5,15],[6],[15,19],[9,12,13]], source = 15, target = 12 输出：-1
提示：
1 <= routes.length <= 500.1 <= routes[i].length <= 105routes[i]中的所有值 互不相同sum(routes[i].length) <= 1050 <= routes[i][j] < 1060 <= source, target < 106
对于本题，我们可以将公交线路看成图中的节点，对于任意两条公交线路，如果它们有公共的公交站点，那么这两个公交线路之间就有一条边。
我们用
接下来我们开始建图。遍历哈希表
接下来，我们可以通过 BFS 求出从
时间复杂度
class Solution:
def numBusesToDestination(
self, routes: List[List[int]], source: int, target: int
) -> int:
if source == target:
return 0
# 一条公交线路有哪些公交站
s = [set(r) for r in routes]
# 一个公交站在哪些公交线路有
d = defaultdict(list)
for i, r in enumerate(routes):
for v in r:
d[v].append(i)
g = defaultdict(list)
for ids in d.values():
m = len(ids)
for i in range(m):
for j in range(i + 1, m):
a, b = ids[i], ids[j]
g[a].append(b)
g[b].append(a)
q = deque(d[source])
ans = 1
vis = set(d[source])
while q:
for _ in range(len(q)):
i = q.popleft()
if target in s[i]:
return ans
for j in g[i]:
if j not in vis:
vis.add(j)
q.append(j)
ans += 1
return -1class Solution {
public int numBusesToDestination(int[][] routes, int source, int target) {
if (source == target) {
return 0;
}
int n = routes.length;
Set<Integer>[] s = new Set[n];
List<Integer>[] g = new List[n];
Arrays.setAll(s, k -> new HashSet<>());
Arrays.setAll(g, k -> new ArrayList<>());
Map<Integer, List<Integer>> d = new HashMap<>();
for (int i = 0; i < n; ++i) {
for (int v : routes[i]) {
s[i].add(v);
d.computeIfAbsent(v, k -> new ArrayList<>()).add(i);
}
}
for (var ids : d.values()) {
int m = ids.size();
for (int i = 0; i < m; ++i) {
for (int j = i + 1; j < m; ++j) {
int a = ids.get(i), b = ids.get(j);
g[a].add(b);
g[b].add(a);
}
}
}
Deque<Integer> q = new ArrayDeque<>();
Set<Integer> vis = new HashSet<>();
int ans = 1;
for (int v : d.get(source)) {
q.offer(v);
vis.add(v);
}
while (!q.isEmpty()) {
for (int k = q.size(); k > 0; --k) {
int i = q.pollFirst();
if (s[i].contains(target)) {
return ans;
}
for (int j : g[i]) {
if (!vis.contains(j)) {
vis.add(j);
q.offer(j);
}
}
}
++ans;
}
return -1;
}
}class Solution {
public:
int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
if (source == target) {
return 0;
}
int n = routes.size();
vector<unordered_set<int>> s(n);
vector<vector<int>> g(n);
unordered_map<int, vector<int>> d;
for (int i = 0; i < n; ++i) {
for (int v : routes[i]) {
s[i].insert(v);
d[v].push_back(i);
}
}
for (auto& [_, ids] : d) {
int m = ids.size();
for (int i = 0; i < m; ++i) {
for (int j = i + 1; j < m; ++j) {
int a = ids[i], b = ids[j];
g[a].push_back(b);
g[b].push_back(a);
}
}
}
queue<int> q;
unordered_set<int> vis;
int ans = 1;
for (int v : d[source]) {
q.push(v);
vis.insert(v);
}
while (!q.empty()) {
for (int k = q.size(); k; --k) {
int i = q.front();
q.pop();
if (s[i].count(target)) {
return ans;
}
for (int j : g[i]) {
if (!vis.count(j)) {
vis.insert(j);
q.push(j);
}
}
}
++ans;
}
return -1;
}
};func numBusesToDestination(routes [][]int, source int, target int) int {
if source == target {
return 0
}
n := len(routes)
s := make([]map[int]bool, n)
g := make([][]int, n)
d := map[int][]int{}
for i, r := range routes {
for _, v := range r {
if s[i] == nil {
s[i] = make(map[int]bool)
}
s[i][v] = true
d[v] = append(d[v], i)
}
}
for _, ids := range d {
m := len(ids)
for i := 0; i < m; i++ {
for j := i + 1; j < m; j++ {
a, b := ids[i], ids[j]
g[a] = append(g[a], b)
g[b] = append(g[b], a)
}
}
}
q := d[source]
vis := map[int]bool{}
for _, v := range d[source] {
vis[v] = true
}
ans := 1
for len(q) > 0 {
for k := len(q); k > 0; k-- {
i := q[0]
q = q[1:]
if s[i][target] {
return ans
}
for _, j := range g[i] {
if !vis[j] {
vis[j] = true
q = append(q, j)
}
}
}
ans++
}
return -1
}public class Solution {
public int NumBusesToDestination(int[][] routes, int source, int target) {
if (source == target) {
return 0;
}
Dictionary<int, HashSet<int>> stopToRoutes = new Dictionary<int, HashSet<int>>();
List<HashSet<int>> routeToStops = new List<HashSet<int>>();
for (int i = 0; i < routes.Length; i++) {
routeToStops.Add(new HashSet<int>());
foreach (int stop in routes[i]) {
routeToStops[i].Add(stop);
if (!stopToRoutes.ContainsKey(stop)) {
stopToRoutes[stop] = new HashSet<int>();
}
stopToRoutes[stop].Add(i);
}
}
Queue<int> queue = new Queue<int>();
HashSet<int> visited = new HashSet<int>();
int ans = 0;
foreach (int routeId in stopToRoutes[source]) {
queue.Enqueue(routeId);
visited.Add(routeId);
}
while (queue.Count > 0) {
int count = queue.Count;
ans++;
for (int i = 0; i < count; i++) {
int routeId = queue.Dequeue();
foreach (int stop in routeToStops[routeId]) {
if (stop == target) {
return ans;
}
foreach (int nextRoute in stopToRoutes[stop]) {
if (!visited.Contains(nextRoute)) {
visited.Add(nextRoute);
queue.Enqueue(nextRoute);
}
}
}
}
}
return -1;
}
}