102. 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

return its level order traversal as:

[  [3],  [9,20],  [15,7]
]
/** * 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;        queue<TreeNode*> current,next;        if(root==NULL)             return result;        else            current.push(root);        while(!current.empty())        {            vector<int> level;//elements in one level            while(!current.empty()){            TreeNode* node=current.front();            current.pop();            level.push_back(node->val);            if(node->left != NULL) next.push(node->left);            if(node->right != NULL) next.push(node->right);        }        result.push_back(level);        swap(next,current);    }    return result;    }};