优化79

Author: diguage

docs/0079-word-search.adoc

@@ -46,10 +46,17 @@ image::images/0079-03.jpg[{image_attr}]
 
 == 思路分析
 
+典型的回溯题目，常规剪枝技巧如下：
+
 image:images/0079-01.png[{image_attr}]
 
 image:images/0079-02.png[{image_attr}]
 
+除了常规剪枝技巧外，还有两个优化点：
+
+. 先判断字母出现次数是否满足要求。网友戏称为可行性剪枝。
+. 如果结尾字母出现次数更少，则将单词翻转，从尾部开始查找，这样可以更少回溯。网友戏称为顺序剪枝。
+
 [[src-0079]]
 [tabs]
 ====

logbook/202503.adoc

@@ -1917,7 +1917,7 @@ endif::[]
 |{counter:codes2503}
 |{leetcode_base_url}/word-search/[79. 单词搜索^]
 |{doc_base_url}/0079-word-search.adoc[题解]
-|✅ 回溯。为了尽可能高效，找到就返回。
+|✅ 回溯。为了尽可能高效，找到就返回。有两个优化点：①先判断字母出现次数是否满足要求；②如果结尾字母出现次数更少，则将单词翻转，从尾部开始查找，这样可以更少回溯。
 
 |===
 

src/main/java/com/diguage/algo/leetcode/_0079_WordSearch_3.java

@@ -8,21 +8,52 @@ public class _0079_WordSearch_3 {
    * @since 2025-11-15 08:52:34
    */
   public boolean exist(char[][] board, String word) {
+    // 优化一：检查字母出现次数
+    char[] counter = new char[52];
+    for (char[] chars : board) {
+      for (char c : chars) {
+        counter[getIndex(c)]++;
+      }
+    }
+    char[] cnt = new char[52];
+    for (char c : word.toCharArray()) {
+      cnt[getIndex(c)]++;
+    }
+    for (int i = 0; i < cnt.length; i++) {
+      if (cnt[i] > counter[i]) {
+        return false;
+      }
+    }
+
+    // 优化二：如果最后字符出现次数更小，则从它开始，回溯次数更少
+    int ti = getIndex(word.charAt(word.length() - 1));
+    int hi = getIndex(word.charAt(0));
+    if (cnt[ti] < cnt[hi]) {
+      word = new StringBuilder(word).reverse().toString();
+    }
+
+    // 回溯查找
     for (int c = 0; c < board.length; c++) {
       for (int r = 0; r < board[c].length; r++) {
-        if (board[c][r] == word.charAt(0)) {
-          boolean found = backtrack(board, word, c, r, 0);
-          if (found) {
-            return true;
-          }
+        boolean found = backtrack(board, c, r, word, 0);
+        if (found) {
+          return true;
         }
       }
     }
     return false;
   }
 
-  private boolean backtrack(char[][] board, String word,
-                            int column, int row, int index) {
+  private int getIndex(char c) {
+    int index = c - 'A';
+    if (c >= 'a') {
+      index = (c - 'a') + 26;
+    }
+    return index;
+  }
+
+  private boolean backtrack(char[][] board, int column, int row,
+                            String word, int index) {
     if (index == word.length()) {
       return true;
     }
@@ -33,22 +64,22 @@ private boolean backtrack(char[][] board, String word,
     }
     board[column][row] = '.';
     // 上
-    boolean found = backtrack(board, word, column - 1, row, index + 1);
+    boolean found = backtrack(board, column - 1, row, word, index + 1);
     if (found) {
       return true;
     }
     // 下
-    found = backtrack(board, word, column + 1, row, index + 1);
+    found = backtrack(board, column + 1, row, word, index + 1);
     if (found) {
       return true;
     }
     // 左
-    found = backtrack(board, word, column, row - 1, index + 1);
+    found = backtrack(board, column, row - 1, word, index + 1);
     if (found) {
       return true;
     }
     // 右
-    found = backtrack(board, word, column, row + 1, index + 1);
+    found = backtrack(board, column, row + 1, word, index + 1);
     if (found) {
       return true;
     }