feat: add solutions to lc/lcof2 problems (#638)

lcof2 No.028 & lc No.0430. Flatten a Multilevel Doubly Linked List

Author: Yousazoe

lcof2/剑指 Offer II 028. 展平多级双向链表/README.md

@@ -187,6 +187,58 @@ class Solution {
 }
 ```
 
+### **C++**
+
+```cpp
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    Node* prev;
+    Node* next;
+    Node* child;
+};
+*/
+
+class Solution {
+public:
+    Node* flatten(Node* head) {
+        flattenGetTail(head);
+        return head;
+    }
+
+    Node* flattenGetTail(Node* head) {
+        Node* cur = head;
+        Node* tail = nullptr;
+
+        while (cur) {
+            Node* next = cur->next;
+            if (cur->child) {
+                Node* child = cur->child;
+                Node* childTail = flattenGetTail(cur->child);
+
+                cur->child = nullptr;
+                cur->next = child;
+                child->prev = cur;
+                childTail->next = next;
+
+                if (next)
+                    next->prev = childTail;
+
+                tail = childTail;
+            } else {
+                tail = cur;
+            }
+
+            cur = next;
+        }
+
+        return tail;
+    }
+};
+```
+
 ### **...**
 
 ```

lcof2/剑指 Offer II 028. 展平多级双向链表/Solution.cpp

@@ -0,0 +1,47 @@
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    Node* prev;
+    Node* next;
+    Node* child;
+};
+*/
+
+class Solution {
+public:
+    Node* flatten(Node* head) {
+        flattenGetTail(head);
+        return head;
+    }
+
+    Node* flattenGetTail(Node* head) {
+        Node* cur = head;
+        Node* tail = nullptr;
+
+        while (cur) {
+            Node* next = cur->next;
+            if (cur->child) {
+                Node* child = cur->child;
+                Node* childTail = flattenGetTail(cur->child);
+
+                cur->child = nullptr;
+                cur->next = child;
+                child->prev = cur;
+                childTail->next = next;
+
+                if (next)
+                    next->prev = childTail;
+
+                tail = childTail;
+            } else {
+                tail = cur;
+            }
+
+            cur = next;
+        }
+
+        return tail;
+    }
+};

solution/0400-0499/0430.Flatten a Multilevel Doubly Linked List/README.md

@@ -169,6 +169,58 @@ class Solution {
 }
 ```
 
+### **C++**
+
+```cpp
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    Node* prev;
+    Node* next;
+    Node* child;
+};
+*/
+
+class Solution {
+public:
+    Node* flatten(Node* head) {
+        flattenGetTail(head);
+        return head;
+    }
+
+    Node* flattenGetTail(Node* head) {
+        Node* cur = head;
+        Node* tail = nullptr;
+
+        while (cur) {
+            Node* next = cur->next;
+            if (cur->child) {
+                Node* child = cur->child;
+                Node* childTail = flattenGetTail(cur->child);
+
+                cur->child = nullptr;
+                cur->next = child;
+                child->prev = cur;
+                childTail->next = next;
+
+                if (next)
+                    next->prev = childTail;
+
+                tail = childTail;
+            } else {
+                tail = cur;
+            }
+
+            cur = next;
+        }
+
+        return tail;
+    }
+};
+```
+
 ### **...**
 
 ```

solution/0400-0499/0430.Flatten a Multilevel Doubly Linked List/README_EN.md

@@ -166,6 +166,58 @@ class Solution {
 }
 ```
 
+### **C++**
+
+```cpp
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    Node* prev;
+    Node* next;
+    Node* child;
+};
+*/
+
+class Solution {
+public:
+    Node* flatten(Node* head) {
+        flattenGetTail(head);
+        return head;
+    }
+
+    Node* flattenGetTail(Node* head) {
+        Node* cur = head;
+        Node* tail = nullptr;
+
+        while (cur) {
+            Node* next = cur->next;
+            if (cur->child) {
+                Node* child = cur->child;
+                Node* childTail = flattenGetTail(cur->child);
+
+                cur->child = nullptr;
+                cur->next = child;
+                child->prev = cur;
+                childTail->next = next;
+
+                if (next)
+                    next->prev = childTail;
+
+                tail = childTail;
+            } else {
+                tail = cur;
+            }
+
+            cur = next;
+        }
+
+        return tail;
+    }
+};
+```
+
 ### **...**
 
 ```

solution/0400-0499/0430.Flatten a Multilevel Doubly Linked List/Solution.cpp

@@ -0,0 +1,47 @@
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    Node* prev;
+    Node* next;
+    Node* child;
+};
+*/
+
+class Solution {
+public:
+    Node* flatten(Node* head) {
+        flattenGetTail(head);
+        return head;
+    }
+
+    Node* flattenGetTail(Node* head) {
+        Node* cur = head;
+        Node* tail = nullptr;
+
+        while (cur) {
+            Node* next = cur->next;
+            if (cur->child) {
+                Node* child = cur->child;
+                Node* childTail = flattenGetTail(cur->child);
+
+                cur->child = nullptr;
+                cur->next = child;
+                child->prev = cur;
+                childTail->next = next;
+
+                if (next)
+                    next->prev = childTail;
+
+                tail = childTail;
+            } else {
+                tail = cur;
+            }
+
+            cur = next;
+        }
+
+        return tail;
+    }
+};