Flatten a Multilevel Doubly Linked List

You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked 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 the example below.

Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.


Input:  1---2---3---4---5---6--NULL          
Output: 1-2-3-7-8-11-12-9-10-4-5-6-NULL
# Definition for a Node.
class Node:
    def __init__(self, val, prev, next, child):
        self.val = val
        self.prev = prev
        self.next = next
        self.child = child

class Solution:
    def flatten(self, head: 'Node') -> 'Node':
        if not head:
            return None
        stack = list()
        cur = head
        while cur:
            if cur.child:
                if cur.next:
                    stack += [cur.next]
                cur.next = cur.child
                cur.next.prev = cur
                cur.child = None
            if not cur.next and stack:
                temp = stack.pop()
                cur.next = temp
                temp.prev = cur
            cur = cur.next
        return head

Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

/ \
2 5
/ \ \
3 4 6

The flattened tree should look like:

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def flatten(self, root: TreeNode) -> None:
        Do not return anything, modify root in-place instead.
        node, stack = root, []
        while node:
            if node.right:
            node.right = node.left
            node.left = None
            if not node.right and stack:
                temp = stack.pop()
                node.right = temp
            node = node.right

Reverse Linked List

Reverse a singly linked list.


Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

As you can seen that recursion implementation is pretty easy to achieve, but iteratively achievement might not. Above are two implementations.

# iteratively
class Solution(object):
    def reverseList(self, head):
        :type head: ListNode
        :rtype: ListNode
        if not head:
            return None
        pionner = head
        while pionner.next:
            old_head = head
            head = pionner.next
            pionner.next = pionner.next.next
            head.next = old_head
        return head
# recursively


有一天我吃完泡面发现洗洁精用光了,我随手用剃须泡洗碗时,会觉得可能要是有一个女朋友就好了。并不是因为有了女朋友就不会吃泡面,也不是因为有了女朋友就会有人帮你洗碗,是我想在洗完碗转身回去时会有个人在那里,等着我一脸神秘地说: “嘿!你猜我刚刚用什么洗的碗?

Daily Temperatures

Given a list of daily temperatures T, return a list such that, for each day in the input, tells you how many days you would have to wait until a warmer temperature. If there is no future day for which this is possible, put 0 instead.

For example, given the list of temperatures T = [73, 74, 75, 71, 69, 72, 76, 73], your output should be [1, 1, 4, 2, 1, 1, 0, 0].

Note: The length of temperatures will be in the range [1, 30000]. Each temperature will be an integer in the range [30, 100].

This quiz is a classical application of the stack data structure. Here we can store indexes of each element which is waiting a bigger element in a stack, and calculate the difference between the current index and stored indexes when the current element is bigger than previous elements. The while loop part is really elegant and tricky, in which judging whether the stack is empty and comparing elements.

class Solution:
    def dailyTemperatures(self, T: List[int]) -> List[int]:
        ans = [0] * len(T)

        stk = list()

        for c_index, value in enumerate(T):
            while stk and T[stk[-1]] < value:
                p_index = stk.pop()
                ans[p_index] = c_index - p_index

        return ans