LeetCode #145 Binary Tree Postorder Traversal

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Program

Given a binary tree, return the postorder traversal of its nodes’ values.

Example

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Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [3,2,1]

Solution1

Method: Depth-first Search (Recursive)

Time Complexity: O(n)

Space Complexity: O(n)

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        def helper(root, nodes):
            if not root:
                return
            helper(root.left, nodes)
            helper(root.right, nodes)
            nodes.append(root.val)

        ret = []
        helper(root, ret)

        return ret

Solution2

Method: Depth-first Search (Iterative)

Time Complexity: O(n)

Space Complexity: O(n)

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []

        stack = [(root, False)]
        ret = []

        while stack:
            node, visited = stack.pop()

            if node:
                if not visited:
                    stack.append((node, True))
                    stack.append((node.right, False))
                    stack.append((node.left, False))
                else:
                    ret.append(node.val)

        return ret