LeetCode 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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

Level order traversal, either use queue/vector.

vector<vector<int>> levelOrder(TreeNode* root) {
        if(!root) return {};   // if root == NULL, return empty vector
        vector<vector<int>> res;
        vector<TreeNode*> nodes;
        nodes.push_back(root);
        while(!nodes.empty()) {
            vector<int> tmp;
            vector<TreeNode*> nextLevel;
            for(int i = 0; i < nodes.size(); ++i) {
                tmp.push_back(nodes[i]->val);
                if(nodes[i]->left) nextLevel.push_back(nodes[i]->left);
                if(nodes[i]->right) nextLevel.push_back(nodes[i]->right);
            }
            res.push_back(tmp);
            tmp.clear();
            nodes.swap(nextLevel);
        }
        return res;
    }