212. Word Search II
原题链接:https://leetcode.com/problems/word-search-ii/description/
这道题目也是在看完 LeetCode 上实现前缀树的文章后推荐的练习题。这道题目初看毫无思路,官方也并未提供解答,还是只能看评论区别人提交的答案了。然后评分最高的就是使用前缀树+回溯法(Backtracking)来解决了:
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
* Created by clearbug on 2018/2/26.
*/
public class Solution {
class TrieNode {
TrieNode[] next = new TrieNode[26];
String word;
char c;
@Override
public String toString() {
return "[c is: " + c + ", word is: " + word + "]";
}
}
public TrieNode buildTrie(String[] words) {
TrieNode root = new TrieNode();
for (String w : words) {
TrieNode p = root;
for (char c : w.toCharArray()) {
int i = c - 'a';
if (p.next[i] == null) {
p.next[i] = new TrieNode();
p.next[i].c = c;
}
p = p.next[i];
}
p.word = w;
}
return root;
}
public static void main(String[] args) {
char[][] board = {
{'o', 'a', 'a', 'n'},
{'e', 't', 'a', 'e'},
{'i', 'h', 'k', 'r'},
{'i', 'f', 'l', 'v'}
};
String[] words = {"oath", "pea", "eat", "rain"};
Solution s = new Solution();
System.out.println(s.findWords(board, words));;
}
public List<String> findWords(char[][] board, String[] words) {
List<String> res = new ArrayList<>();
TrieNode root = buildTrie(words);
for (int i = 0; i < board.length; i++) {
for (int j = 0; j < board[0].length; j++) {
dfs (board, i, j, root, res);
}
}
return res;
}
public void dfs(char[][] board, int i, int j, TrieNode p, List<String> res) {
// print(board, i, j, p, res);
char c = board[i][j];
if (c == '#' || p.next[c - 'a'] == null) return;
p = p.next[c - 'a'];
if (p.word != null) { // found one
res.add(p.word);
p.word = null; // de-duplicate
}
board[i][j] = '#';
if (i > 0) dfs(board, i - 1, j ,p, res);
if (j > 0) dfs(board, i, j - 1, p, res);
if (i < board.length - 1) dfs(board, i + 1, j, p, res);
if (j < board[0].length - 1) dfs(board, i, j + 1, p, res);
board[i][j] = c;
}
public void print(char[][] board, int i, int j, TrieNode p, List<String> res) {
System.out.println("====================================================");
System.out.println("board is: ");
for (int k = 0; k < board[0].length; k++) {
System.out.println(Arrays.toString(board[k]));
}
System.out.println("i is: " + i + ", j is: " + j);
System.out.println("p is: " + p);
System.out.println("res is: " + res);
System.out.println("====================================================");
}
}
回溯法有点绕,所以看解答时不能死扣细节,而是看完代码后在脑子中形成大致思路!