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77 lines (71 loc) · 2.27 KB
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package leetcode.medium;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Deque;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class P_207 {
enum State {
UNVISITED, VISITING, VISITED
}
// Topological sort - Kahn's algorithm
public boolean canFinish(int numCourses, int[][] prerequisites) {
final int[] inDegree = new int[numCourses];
final Map<Integer, List<Integer>> g = new HashMap<>();
for (int[] p : prerequisites) {
g.computeIfAbsent(p[1], v -> new ArrayList<>()).add(p[0]);
inDegree[p[0]]++;
}
final Deque<Integer> q = new ArrayDeque<>();
for (int i = 0; i < numCourses; i++) {
if (inDegree[i] == 0) {
q.offerLast(i);
}
}
int i;
for (i = 0; !q.isEmpty(); i++) {
final int curr = q.removeFirst();
for (int next : g.getOrDefault(curr, Collections.emptyList())) {
if (--inDegree[next] == 0) {
q.offerLast(next);
}
}
}
return i == numCourses;
}
public boolean canFinishDFS(int numCourses, int[][] prerequisites) {
final Map<Integer, List<Integer>> g = new HashMap<>();
for (int[] p : prerequisites) {
g.computeIfAbsent(p[1], v -> new ArrayList<>()).add(p[0]);
}
final State[] state = new State[numCourses];
Arrays.fill(state, State.UNVISITED);
for (int i = 0; i < numCourses; i++) {
if (state[i] == State.UNVISITED) {
if (!dfs(state, g, i)) {
return false;
}
}
}
return true;
}
public static boolean dfs(State[] state, Map<Integer, List<Integer>> g, int u) {
if (state[u] == State.VISITED) {
return true;
}
if (state[u] == State.VISITING) {
return false;
}
state[u] = State.VISITING;
for (int v : g.getOrDefault(u, Collections.emptyList())) {
if (!dfs(state, g, v)) {
return false;
}
}
state[u] = State.VISITED;
return true;
}
}