adding queue based level order traversal

lectures/21_hash_tables_II/README.md

@@ -166,3 +166,4 @@ but requires more work to get right.
 - [Leetcode problem 508: Most Frequent Subtree Sum](https://leetcode.com/problems/most-frequent-subtree-sum/). Solution: [p508_most_frequent_subtree_sum.cpp](../../leetcode/p508_most_frequent_subtree_sum.cpp).
 - [Leetcode problem 225: Implement Stack using Queues](https://leetcode.com/problems/implement-stack-using-queues/). Solution: [p225_stack_using_queues.cpp](../../leetcode/p225_stack_using_queues.cpp).
 - [Leetcode problem 232: Implement Queue using Stacks](https://leetcode.com/problems/implement-queue-using-stacks/). Solution: [p232_queue_using_stacks.cpp](../../leetcode/p232_queue_using_stacks.cpp).
+- [Leetcode problem 102: Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/). Solution: [p102_level_order_traversal.cpp](../../leetcode/p102_level_order_traversal.cpp).

leetcode/p102_level_order_traversal.cpp

@@ -0,0 +1,44 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        vector<vector<int>> result;
+        std::queue<TreeNode*> myQueue;
+        if(root==nullptr){
+            return {};
+        }
+        // initial setup of the queue
+        myQueue.push(root);
+        while(!myQueue.empty()){
+            int size = myQueue.size();
+            vector<int> current_level;
+            // assume every node at the next lever is in the queue, visit them one by one.
+            for(int i=0;i<size;i++){
+                TreeNode* current = myQueue.front();
+                myQueue.pop();
+                if(current!=nullptr){
+                    current_level.push_back(current->val);
+                    // before we get to the next level, push every node of the next level into the queue.
+                    if(current->left!=nullptr){
+                        myQueue.push(current->left);
+                    }
+                    if(current->right!=nullptr){
+                        myQueue.push(current->right);
+                    }
+                }
+            }
+            result.push_back(current_level);
+        }
+        return result;
+    }
+};