Leetcode 102. Binary Tree Level Order Traversal

https://leetcode.com/problems/binary-tree-level-order-traversal/

The trick is to add a parameter to indicate the level.

Solution:

# T:O(n) S:O(n)
class Solution:
    # @param root, a tree node
    # @return a list of lists of integers
    def preorder(self, root, level, res):
        if root:
            if len(res) < level+1: res.append([])
            res[level].append(root.val)
            self.preorder(root.left, level+1, res)
            self.preorder(root.right, level+1, res)
    def levelOrder(self, root):
        res=[]
        self.preorder(root, 0, res)
        return res

Run Time: 68 ms