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).

/**
 * 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>> res;
        queue<TreeNode*> q;
        if(root) q.push(root);
        while(!q.empty()) {
            int size = q.size();
            vector<int> level;
            while(size--) {
                level.push_back(q.front()->val);
                if(q.front()->left) q.push(q.front()->left);
                if(q.front()->right) q.push(q.front()->right);
                q.pop();
            }
            res.push_back(level);
        }
        return res;
    }
};