面试题32 - I：从上到下打印二叉树（C++）

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> levelOrder(TreeNode* root) {
        if(root == nullptr) return {};
        vector<int> res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty())
        {
            int level_node = q.size();
            for(int i = 0; i < level_node; i++)
            {
                TreeNode* node = q.front();
                res.push_back(node->val);
                q.pop();
                if(node->left != nullptr) q.push(node->left);
                if(node->right != nullptr) q.push(node->right);
            }
            
        }
        return res;
    }
};