102. Binary Tree Level Order Traversal Solution using marker

levelOrder.cpp

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result; // array of arrays to store final result 
        vector<int> levelresult;    // array storing result for each level temporarily
        
        queue<TreeNode*> bfs;       // bfs queue
        TreeNode* marker = new TreeNode(INT_MIN);  // dummy node to mark end of level
        
        bfs.push(root);
        bfs.push(marker);
        
        while(!bfs.empty()){
            TreeNode* current = bfs.front(); bfs.pop(); // pop from front or standard dequeue
            
            if(current){
                // if dummy node popped from queue means a level has ended 
                if(current->val == INT_MIN){
                    result.push_back(levelresult); // store level result
                    levelresult = vector<int>(0);  // initialize levelresult for next level
                    
                    // if queue not empty after removing marker, meaning a next level exists or null nodes remaining
                    if(!bfs.empty()) bfs.push(marker);
                }
                else{
                    levelresult.push_back(current->val); // storing current level values into levelresult            
                    
                    // kids pushed at back of queue ( standard enqueue operation )
                    bfs.push(current->left); 
                    bfs.push(current->right);
                }
            }
        }
        
        result.pop_back();
        return result;
    }
};