Level Order Traversal

Problem

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],

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2
3
4
5
  3
 / \
9  20
  /  \
 15   7

return its level order traversal as:

1
2
3
4
5
[
  [3],
  [9,20],
  [15,7]
]

Accepted

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/**
 * 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 {
    vector<vector<int>> nodes;
    void preorder(TreeNode* root, int level) {
        if (root == nullptr) return;
        if (level >= nodes.size()) {
            // create new level
            vector<int> v = {root->val};
            nodes.push_back(v);
        } else {
            nodes[level].push_back(root->val);
        }
        preorder(root->left, level+1);
        preorder(root->right, level+1);

    }
public:

    vector<vector<int>> levelOrder(TreeNode* root) {
        preorder(root, 0);
        return nodes;
    }
};