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62 lines (57 loc) · 1.9 KB
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package leetcode.hard;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Deque;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class P_269 {
static class Node {
int inDegree;
char c;
Node(char c) {
this.c = c;
}
}
public String alienOrder(String[] words) {
final Map<Character, Node> nodes = new HashMap<>();
final Map<Node, List<Node>> g = new HashMap<>();
for (String word : words) {
for (char c : word.toCharArray()) {
nodes.putIfAbsent(c, new Node(c));
}
}
for (int i = 0; i < words.length - 1; i++) {
if (words[i].length() > words[i + 1].length() && words[i].startsWith(words[i + 1])) {
return "";
}
for (int j = 0; j < Math.min(words[i].length(), words[i + 1].length()); j++) {
final char origin = words[i].charAt(j);
final char down = words[i + 1].charAt(j);
if (origin != down) {
g.computeIfAbsent(nodes.get(origin), v -> new ArrayList<>()).add(nodes.get(down));
nodes.get(down).inDegree++;
break;
}
}
}
final StringBuilder sb = new StringBuilder();
final Deque<Node> q = new ArrayDeque<>();
for (Node n : nodes.values()) {
if (n.inDegree == 0) {
q.offerLast(n);
}
}
while (!q.isEmpty()) {
final Node node = q.removeFirst();
sb.append(node.c);
for (Node n : g.getOrDefault(node, Collections.emptyList())) {
if (--n.inDegree == 0) {
q.offerLast(n);
}
}
}
return sb.length() == nodes.size() ? sb.toString() : "";
}
}