0102. Binary Tree Level Order Traversal.md

/**
 * 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) {
        if(!root) return {};

        queue<TreeNode*> q;
        q.push(root);
        vector<vector<int>> res;

        while(!q.empty()) {
            int size = q.size();
            vector<int> arr;
            for(int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();

                arr.push_back(node->val);
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
            res.push_back(arr);
        }

        return res;
    }
};

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Time Complexity: O(n)
Space Complexity: O(n)