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。

思路：用一个队列存左右的节点，然后按顺序取出值。

/*
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) {
        queue<TreeNode*>q;
        vector<int>v;
        TreeNode *node = root;
        v.clear();
        if(root == NULL){
            return v;
        }
        q.push(node);
        while(!q.empty()){
            node = q.front();
            q.pop();
            if(!node){
                continue;
            }
            v.push_back(node->val);       
            q.push(node->left);
            q.push(node->right);
        }
        return v;
    }
};