Update notes

notes/src/SUMMARY.md

@@ -1,4 +1,8 @@
-# Summary
+# 思路
 
 - [深度优先遍历](./dfs.md)
 - [广度优先遍历](./bfs.md)
+
+## 经典代码
+
+- [合并两个有序链表](./merge_two_sorted_linked_lists.md)

notes/src/merge_two_sorted_linked_lists.md

@@ -0,0 +1,18 @@
+# 合并两个有序链表
+
+```cpp
+ListNode* mergeTwoLists(ListNode *a, ListNode *b) {
+    if ((!a) || (!b)) return a ? a : b;
+    ListNode head, *tail = &head, *aPtr = a, *bPtr = b;
+    while (aPtr && bPtr) {
+        if (aPtr->val < bPtr->val) {
+            tail->next = aPtr; aPtr = aPtr->next;
+        } else {
+            tail->next = bPtr; bPtr = bPtr->next;
+        }
+        tail = tail->next;
+    }
+    tail->next = (aPtr ? aPtr : bPtr);
+    return head.next;
+}
+```