# LeetCode: 117. Populating Next Right Pointers in Each Node II

**The Problem**

Given a binary tree

```
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**

`Input: root = [1,2,3,4,5,null,7] Output: [1,#,2,3,#,4,5,7,#] Explanation: Given the above 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.`

**Constraints:**

- The number of nodes in the tree is in the range
`[0, 6000]`

. `-100 <= Node.val <= 100`

**Solution**

We opted for a recursive approach to tackle this problem, and upon evaluating our solution on the LeetCode platform, we achieved the following outcome:

Here's the code that led us to this result.

```
Node* connect(Node* root) {
vector<vector<Node*>> levels;
connectHelper(root, 0, levels);
return root;
}
void connectHelper(Node *&root, int currentLevel, vector<vector<Node*>> &levels) {
if (root == NULL) return;
int nextLevel = currentLevel + 1;
if(levels.size() < nextLevel) levels.resize(nextLevel);
if( levels[currentLevel].size() > 0)
levels[currentLevel].back()->next = root;
levels[currentLevel].push_back(root);
connectHelper(root->left, nextLevel, levels);
connectHelper(root->right, nextLevel, levels);
}
```

Let's break down the code:

1. `connect`

is a function that takes a binary tree represented by the `Node*`

`root`

and returns the same `root`

after connecting the nodes at each level.

2. It initializes a vector of vectors called `levels`

to keep track of nodes at each level.

3. It calls the `connectHelper`

function to perform the actual connection.

4. `connectHelper`

is a recursive function used to traverse the binary tree and connect nodes at the same level.

5. It takes three parameters:

`root`

: A reference to a pointer to the current node in the traversal.`currentLevel`

: An integer representing the current level in the tree.`levels`

: A reference to the vector of vectors that stores nodes at each level.

6. If the `root`

is `NULL`

(indicating an empty subtree), the function returns early.

7. It calculates `nextLevel`

by incrementing `currentLevel`

by 1. This represents the level of the child nodes.

8. It ensures that the `levels`

vector has enough subvectors to accommodate nodes at the current level. If not, it resizes `levels`

accordingly.

9. If there are nodes in the current level (i.e., `levels[currentLevel]`

is not empty), it sets the `next`

pointer of the last node in that level to point to the current `root`

. This connects nodes at the same level.

10. It then adds the current `root`

to the current level's vector in `levels`

.

11. Finally, it recursively calls `connectHelper`

for the left and right child nodes, passing the `nextLevel`

and `levels`

vector by reference to continue the traversal and connection process.