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]
]

Solution

/**
 * 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>> res;
        if ( root == NULL ) return res;
        vector<TreeNode*> res1(1, root);
        while ( res1.size() != 0 ) {
            int nres = res1.size();
            vector<int> tmp;
            vector<TreeNode*> res2;
            for ( int i = 0; i <= nres-1; i++ ) {
                tmp.push_back(res1[i]->val);
                if ( res1[i]->left != NULL ) res2.push_back(res1[i]->left);
                if ( res1[i]->right != NULL ) res2.push_back(res1[i]->right);
            }
            res.push_back(tmp);
            res1 = res2;
        }
        return res;
    }
};