102.cpp

#include <bits/stdc++.h>
#include <queue>
#include <vector>
using namespace std;

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>> res;
        if (!root) return res;

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

        while (!q.empty()) {
            int sz = q.size();
            vector<int> level;

            for (int i=sz; i>0; i--) {
                TreeNode* node = q.front();
                q.pop();
                if (node) {
                    level.push_back(node->val);
                    q.push(node->left);
                    q.push(node->right);
                }
            }

            if (!level.empty())
                res.push_back(level);
        }

        return res;
    }
};