Populating Next Right Pointers in Each Node in O(1) space (without queue and level order)

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Example 1:

Input: root = [1,2,3,4,5,6,7]
Output: [1,#,2,3,#,4,5,6,7,#]
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. The serialized output is in level order as connected by the next pointers, with '#' signifying the end of each level.

Example 2:

Input: root = []
Output: []

class Solution
{
public:
    Node *connect(Node *root)
    {
        Node *p1 = root, *p2;
        while (p1 != NULL)
        {
            p2 = p1;

            while (p2 != NULL)
            {
                if (p2->left)
                    p2->left->next = p2->right;

                if (p2->next && p2->right)
                    p2->right->next = p2->next->left;

                p2 = p2->next;
            }

            p1 = p1->left;
        }
        return root;
    }
};

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