Vertical sum in a given binary tree

I'm working on this problem,

Problem statement,

Given a Binary Tree, find vertical sum of the nodes that are in same vertical line. Print all sums through different vertical lines.

Examples:

     1
   /   \
  2      3
 / \    / \
4   5  6   7

The tree has 5 vertical lines

• Vertical-Line-1 has only one node 4 => vertical sum is 4
• Vertical-Line-2: has only one node 2=> vertical sum is 2
• Vertical-Line-3: has three nodes: 1,5,6 => vertical sum is 1+5+6 = 12
• Vertical-Line-4: has only one node 3 => vertical sum is 3
• Vertical-Line-5: has only one node 7 => vertical sum is 7

So expected output is 4, 2, 12, 3 and 7

I’m wondering if there is a way to not write recursive code:

from collections import defaultdict
class TreeNode:
    def __init__(self, value, left, right):
        self.value = value
        self.left = left
        self.right = right
    def vertical_traverse(self, level):
        stack = []
        result = defaultdict(list) # key: level, value: list of node identifier
        stack.append((self,level))
        while len(stack) > 0:
            node, level = stack.pop(-1)
            result[level].append(node.value)
            if node.left:
                stack.append((node.left, level-1))
            if node.right:
                stack.append((node.right, level+1))
        return result

if __name__ == "__main__":
    root = TreeNode(1, TreeNode(2, TreeNode(4, None, None), TreeNode(5, None, None)), TreeNode(3, TreeNode(6, None, None), TreeNode(7, None, None)))
    result = root.vertical_traverse(0)
    for k,v in result.items():
        print k, sum(v)