LeetCode 117. Populating Next Right Pointers in Each Node II

Description

https://leetcode.com/problems/populating-next-right-pointers-in-each-node-ii/

Given a binary tree

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.

Follow up:

  • You may only use constant extra space.
  • Recursive approach is fine, you may assume implicit stack space does not count as extra space for this problem.

Example 1:

Input: root = [1,2,3,4,5,null,7]
Output: [1,#,2,3,#,4,5,7,#]
Explanation: Given the above binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B. The serialized output is in level order as connected by the next pointers, with '#' signifying the end of each level.

Constraints:

  • The number of nodes in the given tree is less than 6000.
  • -100 <= node.val <= 100

Explanation

Use breadth-first search to link the nodes with their right nodes.

Python Solution

"""
# Definition for a Node.
class Node:
    def __init__(self, val: int = 0, left: 'Node' = None, right: 'Node' = None, next: 'Node' = None):
        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
        
        queue = []        
        queue.append(root)
        
        while queue:
            size = len(queue)
            
            prev = None
            for i in range(size):
                node = queue.pop(0)
                
                if node.right:
                    queue.append(node.right)
                    
                if node.left:
                    queue.append(node.left)
                    
                node.next = prev
                prev = node
                
                
        return root
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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