LeetCode--Binary Tree Level Order Traversal

用两个队列实现，交叉使用，达到判断层的目的

/**
 * Definition for binary tree
 * 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> > res;
        if(root == NULL)
        {
            return res;
        }
        queue<TreeNode *> q1;
        queue<TreeNode *> q2;
        q1.push(root);
        while(!q1.empty() || !q2.empty())
        {
            vector<int> tmpres;
            if(!q1.empty())
            {
                while(!q1.empty())
                {
                    TreeNode *tmp = q1.front();
                    q1.pop();
                    tmpres.push_back(tmp->val);
                    if(tmp->left!=NULL)
                    {
                        q2.push(tmp->left);
                    }
                    if(tmp->right!=NULL)
                    {
                        q2.push(tmp->right);
                    }
                }
            }
            else
            {
                while(!q2.empty())
                {
                    TreeNode *tmp = q2.front();
                    q2.pop();
                    tmpres.push_back(tmp->val);
                    if(tmp->left!=NULL)
                    {
                        q1.push(tmp->left);
                    }
                    if(tmp->right!=NULL)
                    {
                        q1.push(tmp->right);
                    }
                }
            }
            res.push_back(tmpres);
        }
        return res;
    }
};