From 04d2feb0eb12aeb22e7b8a85f51e47e471ef9839 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=94=90=E6=A0=91=E6=A3=AE?= <tangshusen@pku.edu.cn>
Date: Thu, 8 Aug 2019 16:31:16 +0800
Subject: [PATCH] Create 138. Copy List with Random Pointer.md

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+# [138. Copy List with Random Pointer](https://leetcode.com/problems/copy-list-with-random-pointer/)
+# 思路
+题目要求深拷贝一个链表，难点就在于如何处理随机指针的问题。
+（其实这题和[133. Clone Graph](https://leetcode.com/problems/clone-graph/)拷贝图是类似的，都需要用一个HashMap来记录节点与其副本的映射关系。）
+
+## 思路一
+可以先不管random指针，遍历一遍链表并进行拷贝，同时用一个HashMap记录节点与其副本的隐射关系。第二次遍历再处理random指针。
+
+## 思路二
+也可以采用DFS的思路进行递归拷贝，这样就和133. Clone Graph中的DFS解法很类似了。
+
+## 思路三
+前两种方法都需要开辟一个HashMap，如果不允许额外开辟空间呢？
+
+我们可以遍历一遍链表，然后将每个节点后接上其对应副本节点（同样是先不管副本节点的random指针），遍历完成后即变成
+```
+原链表（省略random，下同）:
+1 -> 2 -> 3.
+将每个节点后接上其对应副本节点:
+1 -> 1' -> 2 -> 2' -> 3 -> 3'.
+```
+然后再遍历一遍处理好副本节点的random指针，最后将新的链表拆成新旧两个部分即可。    
+代码这里就不写了，有兴趣可以去试试。
+
+# C++
+## 思路一
+``` C++
+class Solution {
+public:
+    Node* copyRandomList(Node* head) {
+        if(!head) return NULL;
+        unordered_map<Node*, Node*>mp;
+        Node *head_cp = new Node(head -> val);
+        mp[head] = head_cp;
+        Node *p1 = head_cp, *p2 = head -> next;
+        while(p2){
+            p1 -> next = new Node(p2 -> val);
+            mp[p2] = p1 -> next;
+
+            p1 = p1 -> next; p2 = p2 -> next;
+        }
+        p1 -> next = NULL;
+        
+        p1 = head_cp; p2 = head;
+        while(p2){
+            p1 -> random = mp[p2 -> random];
+            p1 = p1 -> next; p2 = p2 -> next;
+        }
+        
+        return head_cp;
+        
+    }
+};
+```
+
+## 思路二
+``` C++
+class Solution {
+private:
+    Node* DFS(Node *node, unordered_map<Node*, Node *>&mp){
+        if(!node) return NULL;
+        if(mp.count(node)) return mp[node];
+        else{
+            Node *node_cp = new Node(node -> val);
+            mp[node] = node_cp;
+            node_cp -> next = DFS(node -> next, mp);
+            node_cp -> random = DFS(node -> random, mp);
+            return node_cp;
+        }
+    }
+public:
+    Node* copyRandomList(Node* head) {
+        if(!head) return NULL;
+        unordered_map<Node*, Node*>mp;
+        return DFS(head, mp);
+        
+    }
+};
+```