connectNodes.cpp

#include <bits/stdc++.h> 
/*
    ----------------- Binary Tree node class for reference -----------------

    template <typename T>
    class BinaryTreeNode {
        public : 
            T data;
            BinaryTreeNode<T> *left;
            BinaryTreeNode<T> *right;
            BinaryTreeNode<T> *next;

            BinaryTreeNode(T data) {
                this -> data = data;
                left = NULL;
                right = NULL;
                next = NULL;
            }
    };
*/

void connectNodes(BinaryTreeNode< int > *root) {
    queue<BinaryTreeNode<int>*> q;
    q.push(root);
    while(q.size()) {
        BinaryTreeNode<int> *pre = NULL;
        int sz = q.size();

        for(int i=0; i<sz; i++) {
            auto cur = q.front();
            q.pop();

            if(pre != NULL)
                pre->next = cur;
            pre = cur;
            if(cur->left)
                q.push(cur->left);
            if(cur->right)
                q.push(cur->right);
        }
    }
}