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Bus_815.java
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Bus_815.java
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import java.util.*;
class Bus_815 {
// My solution by rewriting standard solution with bidirectional BFS
// Very fast (17ms, faster than 99.00%)
// Let N be the number of buses, and number of stops
// Time: O(N^2 lgN), Space: O(N^2)
/*
public int numBusesToDestination(int[][] routes, int S, int T) {
if (S == T) return 0;
int N = routes.length;
boolean[] usedNode = new boolean[N];
List<List<Integer>> graph = new ArrayList<>();
Set<Integer> beginSet = new HashSet<>();
Set<Integer> endSet = new HashSet<>();
// O(N^2 lgN)
for (int i = 0; i < N; i++) {
// O(N lgN)
Arrays.sort(routes[i]);
// O(lgN)
if (Arrays.binarySearch(routes[i], S) >= 0) {
beginSet.add(i);
usedNode[i] = true;
}
// O(lgN)
if (Arrays.binarySearch(routes[i], T) >= 0) {
endSet.add(i);
if (usedNode[i]) return 1;
usedNode[i] = true;
}
// O(N)
graph.add(new ArrayList<>());
for (int j = 0; j < i; j++) {
if (intersect(routes[i], routes[j])) {
graph.get(i).add(j);
graph.get(j).add(i);
}
}
}
// O(N^2)
int step = 2;
while (!beginSet.isEmpty() && !endSet.isEmpty()) {
if (beginSet.size() > endSet.size()) {
Set<Integer> set = beginSet;
beginSet = endSet;
endSet = set;
}
Set<Integer> nextLevel = new HashSet<>();
for (int begin : beginSet) {
for (int r : graph.get(begin)) {
if (endSet.contains(r))
return step;
if (!usedNode[r]) {
usedNode[r] = true;
nextLevel.add(r);
}
}
}
beginSet = nextLevel;
step++;
}
return -1;
}
private boolean intersect(int[] A, int[] B) {
int i = 0, j = 0;
while (i < A.length && j < B.length) {
if (A[i] == B[j]) return true;
if (A[i] < B[j]) i++; else j++;
}
return false;
}
*/
// Rewrite most voted solution
public int numBusesToDestination(int[][] routes, int S, int T) {
if (S == T) return 0;
Map<Integer, List<Integer>> map = new HashMap<>();
for (int i = 0; i < routes.length; i++)
for (int j = 0; j < routes[i].length; j++)
map.computeIfAbsent(routes[i][j], z -> new ArrayList<>()).add(i);
// Bidirectional version
/*
Set<Integer> beginSet = new HashSet<>();
beginSet.add(S);
Set<Integer> endSet = new HashSet<>();
endSet.add(T);
boolean[] usedRoute = new boolean[routes.length];
int step = 0;
while (!beginSet.isEmpty() && !endSet.isEmpty()) {
step++;
if (beginSet.size() > endSet.size()) {
Set<Integer> set = beginSet;
beginSet = endSet;
endSet = set;
}
Set<Integer> nextLevel = new HashSet<>();
for (int begin : beginSet) {
for (int busIdx : map.getOrDefault(begin, new ArrayList<>())) {
if (usedRoute[busIdx]) continue;
usedRoute[busIdx] = true;
for (int stop : routes[busIdx]) {
if (endSet.contains(stop)) return step;
nextLevel.add(stop);
}
}
}
beginSet = nextLevel;
}
return -1;
*/
Deque<Integer> q = new ArrayDeque<>();
q.add(S);
boolean[] usedRoute = new boolean[routes.length];
int step = 0;
while (!q.isEmpty()) {
step++;
int size = q.size();
for (int k = 0; k < size; k++) {
int cur = q.removeFirst();
for (int busIdx : map.getOrDefault(cur, new ArrayList<>())) {
if (usedRoute[busIdx]) continue;
usedRoute[busIdx] = true;
for (int stop : routes[busIdx]) {
if (stop == T) return step;
q.add(stop);
}
}
}
}
return -1;
}
// My first solution using BFS
// I use bus stop instead of route, so this solution is very slow (1399 ms)
/*
public int numBusesToDestination(int[][] routes, int S, int T) {
Set<Integer> beginSet = new HashSet<>();
beginSet.add(S);
Set<Integer> endSet = new HashSet<>();
endSet.add(T);
int N = routes.length;
boolean[] usedRoute = new boolean[N];
int step = 0;
while (!beginSet.isEmpty() && !endSet.isEmpty()) {
if (beginSet.size() > endSet.size()) {
Set<Integer> set = beginSet;
beginSet = endSet;
endSet = set;
}
Set<Integer> nextLevel = new HashSet<>();
for (int begin : beginSet) {
if (endSet.contains(begin))
return step;
for (int i = 0; i < N; i++) {
if (usedRoute[i]) continue;
boolean addAllToNext = false;
for (int stop : routes[i]) {
if (begin == stop) {
usedRoute[i] = true;
addAllToNext = true;
break;
}
}
if (addAllToNext) for (int stop : routes[i]) nextLevel.add(stop);
}
}
beginSet = nextLevel;
step++;
}
return -1;
}
*/
}