leetcode138. Copy List with Random Pointer

Construct a deep copy of the list. 

The linked list is represented in the input/output as a list of n nodes. Each node is represented as a pair of [val, random_index] where:

• val: an integer representing Node.val
• random_index: the index of the node (range from 0 to n-1) that the random pointer points to, or null if it does not point to any node.

Your code will only be given the head of the original linked list.

Example 1:

Input: head = [[7,null],[13,0],[11,4],[10,2],[1,0]]
Output: [[7,null],[13,0],[11,4],[10,2],[1,0]]

解法一：
建一个dict，先对每个点copy一个点存在dict的值里，再对每个dict的值，把next和random加上去。
注意：dic[i]如果没有值就会报错，所以这里用dic.get(i),防止有的点random是空，或者最后一个点next是空。
time: O(2n) 
space： O(n)

"""
# Definition for a Node.
class Node:
    def __init__(self, x: int, next: 'Node' = None, random: 'Node' = None):
        self.val = int(x)
        self.next = next
        self.random = random
"""

class Solution:
    def copyRandomList(self, head: 'Optional[Node]') -> 'Optional[Node]':
        dic = {}
        node = cur = head
        while node:
            dic[node] = Node(node.val)
            node = node.next
        while cur:
            dic[cur].next = dic.get(cur.next)
            dic[cur].random = dic.get(cur.random)
            cur = cur.next
        return dic.get(head)