算法：Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).


For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \

   15   7

class Solution {
public:
    vector> levelOrder(TreeNode* root) {
        vector> levelHolder;
        
        if(root==NULL)
            return levelHolder;
            
        vector* aLvl = new vector();
        deque Q;
        Q.push_back(root);
        Q.push_back(NULL);
        
        while(!Q.empty())
        {
            TreeNode *n = Q.front();
            Q.pop_front();
            
            if(n!=NULL)
            {
                aLvl->push_back(n->val);
                
                if(n->left!=NULL)
                    Q.push_back(n->left);
                if(n->right!=NULL)
                    Q.push_back(n->right);
            }
            else
            {
                levelHolder.push_back(*aLvl);
                aLvl = new vector();
                Q.push_back(NULL);
            }
            
            if(n==NULL && Q.size()==1)
                break;
        }
        
        return levelHolder;
    }
};