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96 lines (89 loc) · 2.9 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.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
public class P_126 {
static Map<Integer, Set<String>> map;
static List<List<String>> res;
static Set<String> wordSet;
public List<List<String>> findLadders(String beginWord, String endWord, List<String> wordList) {
map = new HashMap<>();
res = new ArrayList<>();
wordSet = new HashSet<>(wordList);
int step = 0;
final Deque<String> dq = new ArrayDeque<>(Collections.singletonList(beginWord));
final Set<String> seen = new HashSet<>(Collections.singletonList(beginWord));
boolean found = false;
outer:
while (!dq.isEmpty()) {
final Set<String> curr = new HashSet<>();
for (int size = dq.size(); size > 0; size--) {
final String u = dq.removeFirst();
curr.add(u);
if (u.equals(endWord)) {
found = true;
break outer;
}
for (String v : nextLevel(u, seen)) {
dq.offer(v);
}
}
map.put(step++, curr);
}
if (!found) {
return res;
}
final List<String> list = new ArrayList<>(Collections.singletonList(endWord));
dfs(endWord, step, list);
return res;
}
private static void dfs(String u, int level, List<String> list) {
if (level == 0) {
final List<String> copy = new ArrayList<>(list);
Collections.reverse(copy);
res.add(copy);
return;
}
final Set<String> seen = new HashSet<>();
for (String v : map.get(level - 1)) {
if (isOkay(v, u) && seen.add(v)) {
list.add(v);
dfs(v, level - 1, list);
list.remove(list.size() - 1);
}
}
}
private static Set<String> nextLevel(String s, Set<String> seen) {
final Set<String> res = new HashSet<>();
final int n = s.length();
for (int i = 0; i < n; i++) {
final int c = s.charAt(i);
for (char nc = 'a'; nc <= 'z'; nc++) {
if (nc == c) {
continue;
}
final String u = s.substring(0, i) + nc + s.substring(i + 1);
if (wordSet.contains(u) && seen.add(u)) {
res.add(u);
}
}
}
return res;
}
private static boolean isOkay(String l, String r) {
final int n = l.length();
int d = 0;
for (int i = 0; i < n; i++) {
if (l.charAt(i) != r.charAt(i)) {
d++;
}
}
return d == 1;
}
}