LeetCode #116 Populating Next Right Pointers in Each Node

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Problem

Given a binary tree

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struct TreeLinkNode {
  TreeLinkNode *left;
  TreeLinkNode *right;
  TreeLinkNode *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

Given the following perfect binary tree,

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     1
   /  \
  2    3
 / \  / \
4  5  6  7

After calling your function, the tree should look like:

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     1 -> NULL
   /  \
  2 -> 3 -> NULL
 / \  / \
4->5->6->7 -> NULL

Note

  • You may only use constant extra space.
  • Recursive approach is fine, implicit stack space does not count as extra space for this problem.
  • You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

Solution

Method: Breadth-first Search

Time Complexity:

Space Complexity:

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# Definition for binary tree with next pointer.
# class TreeLinkNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#         self.next = None

class Solution:
    # @param root, a tree link node
    # @return nothing
    def connect(self, root):
        level = [root]
        
        while level:
            new_level = []
            idx = 0
            while idx < len(level):
                if level[idx]:
                    if idx + 1 < len(level):
                        level[idx].next =level[idx + 1]
                    new_level.append(level[idx].left)
                    new_level.append(level[idx].right)
                idx += 1

            level = new_level