116. Populating Next Right Pointers in Each Node

by Duct Tape Programmer

Question

Given a binary tree

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.

Note:

You may only use constant extra space.
You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).
For example,
Given the following perfect binary tree,

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

After calling your function, the tree should look like:

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

Quick Hints

  • Traverse level by level
  • Fixing the next pointer
  • Notice that the other side subtree can be reached by curNode's next.

Solution

  • The idea is to generate the next pointer level by level
  • Generating it for the child of same node is similar but we go across tree it becomes a little different.
  • Although at that point we already have the next for the node setup which allow us to go to the other side and use it.

Time complexity

O (n)

Space complexity

O(1)

Notes