LeetCode 116: Populating Next Right Pointers in Each Node

Difficulty: 3

Frequency: 3

Problem:

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

Solution:

/**
 * Definition for binary tree with next pointer.
 * struct TreeLinkNode {
 *  int val;
 *  TreeLinkNode *left, *right, *next;
 *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
 * };
 */
class Solution {
public:
    void connect(TreeLinkNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        connectPerfectRecursive(root);
    }
    void connectPerfectRecursive(TreeLinkNode * root)
    {
        if (root==NULL)
            return;
            
        if (root->left==NULL)
        {
            root->next = NULL;
            return;
        }
        
        connectPerfectRecursive(root->left);
        connectPerfectRecursive(root->right);
        
        TreeLinkNode *left = root->left, *right = root->right;
        while(left!=NULL)
        {
            left->next = right;
            left = left->right;
            right = right->left;
        }
    }
};

Notes:

Although my code passed all tests, I still don't understand the requirement: you may only use constant extra space. In this case, I think tree traverse is necessary. Tree traverse is either recursive or using a stack. Neither of them meets the constant extra space requirement. I will consult others' codes later.