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使用队列 queue, 取第一个是queue.front();

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        if(root == NULL) return result;
        queue<TreeNode *> q;
        q.push(root);
        while(!q.empty()){
            int size = q.size();
            vector<int> cur;
            for(int i = 0; i < size; i++){
                TreeNode * t = q.front();
                q.pop();
                cur.push_back(t->val);
                if(t->left){
                    q.push(t->left);
                }
                if(t->right){
                    q.push(t->right);
                }
            }
            result.push_back(cur);
        }
        return result;
    }
};