diff --git a/省选准备/assets/day2/AC-buildFail.cpp b/省选准备/assets/day2/AC-buildFail.cpp
new file mode 100644
index 0000000..446b8dd
--- /dev/null
+++ b/省选准备/assets/day2/AC-buildFail.cpp
@@ -0,0 +1,20 @@
+void buildFail()
+{
+    int h = 0, t = 0;
+    root->fail = &virt;
+    que[t++] = root;
+    while (h < t)
+    {
+        node *cur = que[h++];
+        for (int i = 0; i < 26; i++)
+        {
+            node *f = cur->fail;
+            while (f->trans[i] == 0) f = f->fail;
+            f = f->trans[i];
+            if (cur->trans[i])
+                (que[t++] = cur->trans[i])->fail = f;
+            else
+                cur->trans[i] = f;
+        }
+    }
+}
\ No newline at end of file
diff --git a/省选准备/省选基础算法.pdf b/省选准备/省选基础算法.pdf
index b5129ac..2c7c50c 100644
Binary files a/省选准备/省选基础算法.pdf and b/省选准备/省选基础算法.pdf differ
diff --git a/省选准备/省选基础算法.tex b/省选准备/省选基础算法.tex
index e3d6a04..9675f90 100644
--- a/省选准备/省选基础算法.tex
+++ b/省选准备/省选基础算法.tex
@@ -158,4 +158,15 @@ Additionally, there are several pairs of people conducting adulterous relationsh
 \subparagraph{\href{http://poj.org/problem?id=2945}{POJ2945} - Find the Clones} 
 $n$个基因片段，每个长度为$m$，输出$n$行表示重复出现$i$次$(1 \leq i \leq n)$的基因片段的个数
 \codeinput[Find the Clones]{assets/day2/poj2945.cpp}
+待续
+\subsection{Aho–Corasick Automaton}
+\paragraph{简介}
+	多模式串字符串匹配，Trie上的KMP，其中$next$数组变成了$fail$指针，功能相同。
+\paragraph{实现}
+	Trie的实现上文已经出现，所以此处不再重复。
+\codeinput[buildFail]{assets/day2/AC-buildFail.cpp}
+\paragraph{练习题}
+\subparagraph{\href{http://acm.hdu.edu.cn/showproblem.php?pid=2222}{HDU2222} - Keywords Search}
+AC 自动机模板题，注意统计答案时，每个节点只能统计一次不要重复统计。
+\codeinput[Keywords Search]{assets/day2/poj3630.cpp}
 \end{document}
\ No newline at end of file