【剑指offer】从上至下打印二叉树

/*
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) {
        vector<int> v;
        if(root == NULL)
        {
            return v;
        }
        queue<TreeNode*> q;    //借助队列来从上至下、从左至右打印
        q.push(root);
        while(!q.empty())
        {
            v.push_back(q.front()->val);
            if(q.front()->left != NULL)    
            {
                q.push(q.front()->left);
            }
            if(q.front()->right != NULL)
            {
                q.push(q.front()->right);
            }
            q.pop();
        }
        return v;
    }
};