从上往下打印二叉树

/*
struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};*/
//就是二叉树的层次遍历，如果要求从下往上打印的话，用reverse函数颠倒一下。
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        vector<int> ans;
        TreeNode* t;
        if(root==NULL)
            return ans;
        queue<TreeNode*> p;
        p.push(root);
        
        while(!p.empty())
        {
            t=p.front();
            ans.push_back(t->val);
            if(t->left!=NULL)
                p.push(t->left);
            if(t->right!=NULL)
                p.push(t->right);
            p.pop();
        }
        return ans;
    }
};