面试常考题，中序遍历用BFS。

/**
 * 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<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>> res;

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

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

            while (size -- )                //遍历该层的所有节点
            {
                TreeNode* t = q.front();
                q.pop();

                if (!t) continue;           //跳过空节点

                level.push_back(t->val);    //push本层节点的值到数组
                q.push(t->left);            //储存下一层节点到队列
                q.push(t->right);
            }

            if (level.size()) 
                res.push_back(level);       //push入该层的所有节点
        }

        return res;
    }
};