Populating Next Right Pointers in Each Node LeetCode Solution

Problem – Populating Next Right Pointers in Each Node LeetCode Solution

You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition:

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.

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: []

Constraints:

  • The number of nodes in the tree is in the range [0, 212 - 1].
  • -1000 <= Node.val <= 1000

Follow-up:

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

Populating Next Right Pointers in Each Node LeetCode Solution in C++

class Solution {
public:
    Node* connect(Node* root) {
        if(!root) return nullptr;
        queue<Node*> q;
        q.push(root);        
        while(size(q)) {
            Node* rightNode = nullptr;                    // set rightNode to null initially
            for(int i = size(q); i; i--) {                // traversing each level
                auto cur = q.front(); q.pop();            // pop a node from current level and,
                cur -> next = rightNode;                  // set its next pointer to rightNode
                rightNode = cur;                          // update rightNode as cur for next iteration
                if(cur -> right)                          // if a child exists
                    q.push(cur -> right),                 // IMP: push right first to do right-to-left BFS
                    q.push(cur -> left);                  // then push left
            }
        }
        return root;
    }
};

Populating Next Right Pointers in Each Node LeetCode Solution in Python

class Solution:
    def connect(self, root):
        if not root: return None
        q = deque([root])
        while q:
            rightNode = None
            for _ in range(len(q)):
                cur = q.popleft()
                cur.next, rightNode = rightNode, cur
                if cur.right:
                    q.extend([cur.right, cur.left])
        return root

Populating Next Right Pointers in Each Node LeetCode Solution in Java

class Solution {
    public Node connect(Node root) {
        if(root == null) return null;
        Queue<Node> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()) {
            Node rightNode = null;
            for(int i = q.size(); i > 0; i--) {
                Node cur = q.poll();
                cur.next = rightNode;
                rightNode = cur;
                if(cur.right != null) {
                    q.offer(cur.right);
                    q.offer(cur.left);
                }
            }
        }
        return root;        
    }
}
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