add word search 2 solution

Author: Tessa1217

word-search-ii/Tessa1217.java

@@ -0,0 +1,90 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+
+    private int[] dx = {0, 0, 1, -1};
+    private int[] dy = {1, -1, 0, 0};
+
+    private int m;
+    private int n;
+
+    class TrieNode {
+        // 단어는 영어 소문자만으로 이루어짐
+        TrieNode[] nodes = new TrieNode[26];
+        String word = null;
+    }
+
+    private void insert(TrieNode root, String word) {
+        TrieNode node = root;
+        for (char c : word.toCharArray()) {
+            int idx = c - 'a';
+            if (node.nodes[idx] == null) {
+                node.nodes[idx] = new TrieNode();
+            }
+            node = node.nodes[idx];
+        }
+        // 단어
+        node.word = word;
+    }
+
+    public List<String> findWords(char[][] board, String[] words) {
+
+        List<String> existingWords = new ArrayList<>();
+
+        // 탐색 최적화를 위해 TrieNode 생성
+        TrieNode root = new TrieNode();
+        for (String word : words) {
+            insert(root, word);
+        }
+
+        m = board.length;
+        n = board[0].length;
+
+        for (int i = 0; i < m; i++) {
+            for (int j = 0; j < n; j++) {
+                dfs(board, i, j, root, existingWords);
+            }
+        }
+
+        return existingWords;
+    }
+
+    private void dfs(char[][] board, int i, int j, TrieNode node, List<String> existingWords) {
+
+        char c = board[i][j];
+
+        // 방문 처리 되었거나 TrieNode에 없는 글자라면 탐색 X => Pruning
+        if (c == '#' || node.nodes[c - 'a'] == null) {
+            return;
+        }
+
+        node = node.nodes[c - 'a'];
+
+        // node의 완성된 단어가 있다면
+        if (node.word != null) {
+            existingWords.add(node.word);
+            node.word = null;
+        }
+
+        // 방문 처리
+        board[i][j] = '#';
+
+        for (int idx = 0; idx < 4; idx++) {
+            int ni = i + dx[idx];
+            int nj = j + dy[idx];
+            if (isRange(ni, nj)) {
+                dfs(board, ni, nj, node, existingWords);
+            }
+        }
+
+        board[i][j] = c;
+    }
+
+    // 탐색 범위 조건
+    private boolean isRange(int i, int j) {
+        return i >= 0 && j >= 0 && i < m && j < n;
+    }
+
+}
+