Given a binary tree. Find the postorder traversal of the tree without using recursion.
Example 1
Input:
1
/ \
2 3
/ \
4 5
Output: 4 5 2 3 1
Explanation:
Postorder traversal (Left->Right->Root) of
the tree is 4 5 2 3 1.
Example 2
Input:
8
/ \
1 5
\ / \
7 10 6
\ /
10 6
Output: 10 7 1 6 10 6 5 8
Explanation:
Postorder traversal (Left->Right->Root)
of the tree is 10 7 1 6 10 6 5 8 .

Your task:
You don't need to read input or print anything. Your task is to complete the function postOrder() which takes the root of the tree as input and returns a list containing the postorder traversal of the tree, calculated without using recursion.

Expected time complexity: O(N)
Expected auxiliary space: O(N)

Constraints:
1 <= Number of nodes <= 10^{5}
1 <= Data of a node <= 10^{5}

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