212. Word Search II
刷
June-21-2019
Trie + word search是CS1501的作业题,当时做了1个月。
DFS + backtrack + trie,正好复习了一下dfs + backtrack
说说做错的地方:
一开始是用board[][]建字典,枚举所有可能,然后在里面搜索words,这样建树花时间太长太长了,相当于枚举出board[][]可以组成的所有单词= =。正确的方法是,通过words建树,然后在board里找,很多情况都可以直接剪枝。
第二个错的地方是,Trie完全没吊用。。记住Root就行,所以一个class足够。然后isLetter其实是说isWord,刚才几个题也写错了,不知道在想什么。
第三个是,DFS是下去的那层来判断是否找到word,要在边界check之前完成;
第四个是,不一定非要boolean[][]可以通过+26变大写之类的来代表visited
另外:
上下左右用了个小trick
字母类还是Node[26]好一些
DFS回溯没做错= =不容易
class Solution {
// 1 0 -1 0
private static final int[] MOVE = new int[]{0, 1, 0, -1, 0};
public class Node {
Character letter;
boolean isLetter;
Node[] nextLevel;
public Node(Character c) {
this.letter = c;
this.isLetter = false;
nextLevel = new Node[26];
}
}
public class Trie {
Node root;
public Trie() {
root = new Node(null);
}
public void add(String word) {
Node temp = root;
for (char c : word.toCharArray()) {
if (temp.nextLevel[c - 'a'] == null) {
temp.nextLevel[c - 'a'] = new Node(c);
}
temp = temp.nextLevel[c - 'a'];
}
temp.isLetter = true;
}
public boolean contains(String word) {
Node temp = root;
for (char c : word.toCharArray()) {
if (null == temp.nextLevel[c - 'a']) {
return false;
} else {
temp = temp.nextLevel[c - 'a'];
}
}
return temp.isLetter;
}
}
public List<String> findWords(char[][] board, String[] words) {
Trie dict = new Trie();
for (String word : words) {
dict.add(word);
}
List<String> result = new ArrayList<>();
boolean[][] visited = new boolean[board.length][board[0].length];
for (int i = 0; i < board.length; i ++) {
for (int j = 0; j < board[0].length; j ++) {
contains(dict.root, i, j, visited, board, new StringBuilder(), result);
}
}
return result;
}
public void contains(Node temp, int i, int j, boolean[][] visited, char[][] board, StringBuilder sb, List<String> res) {
if (temp.isLetter && !res.contains(sb.toString())) {
res.add(sb.toString());
}
if (i < 0 || i >= visited.length || j < 0 || j >= visited[0].length || visited[i][j]) return;
char tempC = board[i][j];
if (temp.nextLevel[tempC - 'a'] == null) return;
temp = temp.nextLevel[tempC - 'a'];
visited[i][j] = true;
sb.append(board[i][j]);
int length = sb.length();
for (int m = 0; m < 4; m ++) {
int horizontal = MOVE[m];
int vertical = MOVE[m + 1];
contains(temp, i + horizontal, j + vertical, visited, board, sb, res);
sb.setLength(length);
}
visited[i][j] = false;
}
}