树的广度优先遍历

struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};

    vector<int> PrintFromTopToBottom(TreeNode* root) {//实现广度优先遍历
        
        vector<int> res;
        if(root==NULL) return res;
        queue<TreeNode*> qu;//创建队列
        qu.push(root);//根节点放入队列
        while(!qu.empty())//当前队列不为空
        {
            res.push_back(qu.front()->val);
            if(qu.front()->left)
                qu.push(qu.front()->left);
            if(qu.front()->right)
                qu.push(qu.front()->right);          
            qu.pop();//当前队头元素出队
        }
        return res;
    }