LeetCode 102. Binary Tree Level Order Traversal

level order

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] ]

answer:

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<TreeNode*> pre;
        vector<vector<int>> result;
        pre.push_back(root);

        while(!pre.empty()){
            myLevel(pre,result);
        }
        return result;
    }
    
    void myLevel(vector<TreeNode*> & pre,vector<vector<int>> & result){
        vector<int> level;
        vector<TreeNode*> temp;
        for(int i = 0; i < pre.size(); i ++){
            TreeNode * root = pre[i];
            if(root){
                level.push_back(root->val);
                temp.push_back(root->left);
                temp.push_back(root->right);
            }
        }
        if(!level.empty())
            result.push_back(level);
        pre = temp;
        return;
    }
};