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Day 1: 102. Binary Tree Level Order Traversal 

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

Before we dive in,

Breathe

Nothing is ever that deep.

Now, let’s

But there is a little trick that I am about to teach you….

We’re going to leverage recursion to navigate through the tree and a map to store the nodes level by level, where the key is the level and the value is an array with the nodes in that level.

We are simply going to iterate through the tree, store in our map the current node we are processing and their level:

treeNodes[level].push_back(root->val);

And every time we call our recursive function, we’re simply going to add 1 to our level so that when we traverse deeper into the tree, we know exactly where we are:

auxLevelOrder(root->left, level + 1); 
auxLevelOrder(root->right, level + 1);

This approach allows us to group nodes together by their levels as we go down the tree. Here’s the complete solution so you can see how it all comes together:

Complete code:

class Solution {
public:
    map<int, vector<int>> treeNodes; // The key is the level, the value is a vector of node values at that level

    void auxLevelOrder(TreeNode* root, int level) {
        if (root != NULL) {
            // Store the current node's value at its corresponding level
            treeNodes[level].push_back(root->val);
            
            // Recurse into the left subtree
            auxLevelOrder(root->left, level + 1);

            // Recurse into the right subtree
            auxLevelOrder(root->right, level + 1);
        }
    }

    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> answer;
        
        // Start the recursive level order traversal from the root at level 0
        auxLevelOrder(root, 0);
        
        // For each level in the map, add its corresponding node values to the final answer
        for (auto level : treeNodes) {
            answer.push_back(level.second);
        }

        return answer;
    }
};

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