[Tree]116. Populating Next Right Pointers in Each Node

• 分类：Tree
• 时间复杂度: O(n)
• 空间复杂度: O(h)

116. Populating Next Right Pointers in Each Node

You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition:

struct Node {

 int val;

 Node *left;

 Node *right;

 Node *next;

}

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Example:

image

Input: {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":null,"right":null,"val":4},"next":null,"right":{"$id":"4","left":null,"next":null,"right":null,"val":5},"val":2},"next":null,"right":{"$id":"5","left":{"$id":"6","left":null,"next":null,"right":null,"val":6},"next":null,"right":{"$id":"7","left":null,"next":null,"right":null,"val":7},"val":3},"val":1}

Output: {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":{"$id":"4","left":null,"next":{"$id":"5","left":null,"next":{"$id":"6","left":null,"next":null,"right":null,"val":7},"right":null,"val":6},"right":null,"val":5},"right":null,"val":4},"next":{"$id":"7","left":{"$ref":"5"},"next":null,"right":{"$ref":"6"},"val":3},"right":{"$ref":"4"},"val":2},"next":null,"right":{"$ref":"7"},"val":1}

Explanation: Given the above perfect binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B.

Note:

• You may only use constant extra space.

• Recursive approach is fine, implicit stack space does not count as extra space for this problem.

代码：

我自己的代码思路：

"""
# Definition for a Node.
class Node:
    def __init__(self, val, left, right, next):
        self.val = val
        self.left = left
        self.right = right
        self.next = next
"""
class Solution:
    
    prev_nodes=[]
    
    def connect(self, root: 'Node') -> 'Node':
        
        if not root:
            return None
        
        return self.helper(root,0)
        
        
    def helper(self,root,level):
        
        if root==None:
            if self.prev_nodes==[]:
                self.prev_nodes=[None] * level
            return
        
        self.helper(root.right,level+1)
        self.helper(root.left,level+1)
        root.next=self.prev_nodes[level]
        self.prev_nodes[level]=root
        return root

youtube上猫打球的思路代码：

"""
# Definition for a Node.
class Node:
    def __init__(self, val, left, right, next):
        self.val = val
        self.left = left
        self.right = right
        self.next = next
"""
class Solution:
    
    prev_nodes=[]
    
    def connect(self, root: 'Node') -> 'Node':
        
        if not root:
            return None
        
        return self.helper(root,0)
        
        
    def helper(self,root,level):
        
        if root==None:
            if self.prev_nodes==[]:
                self.prev_nodes=[None] * level
            return
        
        self.helper(root.right,level+1)
        self.helper(root.left,level+1)
        root.next=self.prev_nodes[level]
        self.prev_nodes[level]=root
        return root

GTH思路代码的变种：

"""
# Definition for a Node.
class Node:
    def __init__(self, val, left, right, next):
        self.val = val
        self.left = left
        self.right = right
        self.next = next
"""
class Solution:
    
    def connect(self, root: 'Node') -> 'Node':
        
        if not root:
            return None
        
        res=Node(0,None,None,None)
        res.left=root
        
        while root!=None:
            self.helper(root)
            root=root.left
        
        return res.left
        
        
    def helper(self,root):
        if root.left==None and root.right==None:
            return
        root.left.next=root.right
        if root.next==None:
            root.right.next==None
        else:  
            root.right.next=root.next.left
        
        if root.next!=None:
            self.helper(root.next)
        else:
            return 

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讨论：

1.自己写了一个代码，但是为啥这么慢？
2.youtube上的小姐姐讲的很不错，但是为啥还是这么慢？
3.用GTH的思路代码为什么还是这么慢？有毒吧