> 考察二叉树的宽度遍历算法，在这使用了queue来实现

queue.cpp

//Runtime: 8 ms, faster than 100.00% 
//Memory Usage: 13.9 MB, less than 55.99%

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        if(!root) return result;
        queue<TreeNode*> q;
        q.push(root);
        q.push(NULL);
        vector<int> cur_vec;
        while(!q.empty()){
            TreeNode* t = q.front();
            q.pop();
            if(t == NULL){
                result.push_back(cur_vec);
                cur_vec.resize(0);
                if(q.size() > 0) 
                    q.push(NULL);
            }else {
                cur_vec.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
        }
        return result;
    }
};