Added Flatten Link List

Author: Yan Cao

Interviews/Flatten_a_Multilevel_Linked_List.py

@@ -0,0 +1,28 @@
+"""
+Given a linked list where in addition to the next pointer, each node has a child pointer, which may or may not point to a separate list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in below figure.You are given the head of the first level of the list. Flatten the list so that all the nodes appear in a single-level linked list. You need to flatten the list in way that all nodes at first level should come first, then nodes of second level, and so on.
+
+Each node is a C struct with the following definition.
+
+![pic](./img/flattenList.png)
+"""
+
+class List():
+    def __init__(self, data):
+        self.data = data
+        self.next = None
+        self.child = None
+
+def flatten_list(head):
+    tail = head
+    while tail.next:
+        tail = tail.next
+
+    while head.next:
+        if head.child:
+            tail.next = head.child
+            while tail.next:
+                tail = tail.next
+        head = head.next
+
+# Note:
+# 题目22 23行之间用了一个tmp, 但我感觉没必要用...

Interviews/Flattening_a_Linked_List.py

@@ -0,0 +1,48 @@
+"""
+Given a linked list where every node represents a linked list and contains two pointers of its type:
+(i) Pointer to next node in the main list (we call it ‘right’ pointer in below code)
+(ii) Pointer to a linked list where this node is head (we call it ‘down’ pointer in below code).
+All linked lists are sorted. See the following example
+```
+       5 -> 10 -> 19 -> 28
+       |    |     |     |
+       V    V     V     V
+       7    20    22    35
+       |          |     |
+       V          V     V
+       8          50    40
+       |                |
+       V                V
+       30               45
+```
+"""
+
+class Node():
+    def __init__(self, data):
+        self.right = None
+        self.down = None
+        self.data = data
+
+def flatten(node):
+    if not node or not node.right:
+        return node
+
+    return merge(node, flatten(node.right))
+
+def merge(node1, node2):
+    if not node1:
+        return node2
+    if not node2:
+        return node1
+
+    if node1.data < node2.data:
+        head = node1
+        head.next = merge(node1.next, node2)
+    else:
+        head = node2
+        head.next = merge(node1, node2.next)
+    return head
+
+# Notice:
+# This is very qiao miao. Using recursion is like DFS, so will first flatten from the end.
+# When we want to flatten the front part, the back parts are already flattened

coding_index.md

@@ -13,6 +13,7 @@
 * [x] Combination Sum II
 * [x] Letter Combination of a Phone Number
 * [ ] 找零钱
+
 -----
 
 * [x] Subset