update

hard/212.word-search-ii.java

@@ -0,0 +1,74 @@
+import java.util.LinkedList;
+
+class Solution {
+    private int[] dy = new int[]{0, 1, 0, -1};
+    private int[] dx = new int[]{1, 0, -1, 0};
+
+    private char[][] board;
+    private int n;
+    private int m;
+    private TrieNode root;
+
+    public List<String> findWords(char[][] board, String[] words) {
+        LinkedList<String> res = new LinkedList<>();
+        if ((n = board.length) == 0 || (m = board[0].length) == 0 || words.length == 0) return res;
+        this.board = board;
+        root = new TrieNode();
+        root.cnt = -1;
+        for (String word : words) root.insert(word);
+        boolean[][] used = new boolean[n][m];
+        for (int y = 0; y < n; y++) {
+            for (int x = 0; x < m; x++) {
+                StringBuilder sb = new StringBuilder();
+                backtrack(y, x, root, sb, used, res);
+            }
+        }
+        return res;
+    }
+
+    private void backtrack(int y, int x, TrieNode curr, StringBuilder sb, boolean[][] used, LinkedList<String> res) {
+        if (curr == null || curr.cnt == 0) return;
+        if (curr.isKey) {
+            res.add(sb.toString());
+            root.remove(sb.toString());
+        }
+        if (y < 0 || n <= y || x < 0 || m <= x || used[y][x]) return;
+        char c = board[y][x];
+        sb.append(c);
+        used[y][x] = true;
+        curr = curr.next[(int) c - 'a'];
+        for (int i = 0; i < 4; i++) {
+            int ny = y + dy[i], nx = x + dx[i];
+            backtrack(ny, nx, curr, sb, used, res);
+        }
+        sb.deleteCharAt(sb.length() - 1);
+        used[y][x] = false;
+    }
+
+    class TrieNode {
+        int cnt;
+        boolean isKey;
+        TrieNode[] next = new TrieNode[26];
+
+        void insert(String str) {
+            TrieNode curr = this;
+            for (char c : str.toCharArray()) {
+                int i = (int) c - 'a';
+                if (curr.next[i] == null) curr.next[i] = new TrieNode();
+                curr = curr.next[i];
+                curr.cnt++;
+            }
+            curr.isKey = true;
+        }
+
+        void remove(String str) {
+            TrieNode curr = this;
+            for (char c : str.toCharArray()) {
+                int i = (int) c - 'a';
+                curr = curr.next[i];
+                curr.cnt--;
+            }
+            curr.isKey = false;
+        }
+    }
+}