23 从上往下打印二叉树

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

C++:

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        if (root == NULL)
            return vector<int>() ;
        queue<TreeNode*> q ;
        vector<int> res ;
        q.push(root) ;
        while(!q.empty()){
            TreeNode* node = q.front() ;
            q.pop() ;
            res.push_back(node->val) ;
            if (node->left != NULL)
                q.push(node->left) ;
            if (node->right != NULL)
                q.push(node->right) ;
        }
        return res ;
    }
};