Binary Tree Postorder Traversal

最新推荐文章于 2022-07-19 11:52:33 发布

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本文介绍了一种实现二叉树后序遍历的方法，包括递归和非递归两种方式。递归方法通过左子树、右子树到根节点的顺序获取遍历结果；非递归方法使用栈来辅助完成遍历过程。

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

Example
Given binary tree {1,#,2,3},

return [3,2,1].

Python:

    """
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
    self.val = val
    self.left, self.right = None, None
"""


class Solution:
"""
@param root: The root of binary tree.
@return: Postorder in ArrayList which contains node values.
"""
'''
# divide and conquer
def postorderTraversal(self, root):
    result = []
    if root is None:
        return result

    left = self.postorderTraversal(root.left)
    right = self.postorderTraversal(root.right)

    result.extend(left)
    result.extend(right)
    result.append(root.val)
    return result
'''    

'''
# Tranverse
def postorderTraversal(self, root):
    result = []
    self.helper(root, result)
    return result

def helper(self, root, result):
    if root is None:
        return
    self.helper(root.left, result)
    self.helper(root.right, result)
    result.append(root.val)
'''

# non recursion
def postorderTraversal(self, root):
    result = []
    if root is None:
        return result
    stack = [root]
    used = set()
    while stack:
        node = stack.pop()
        if node in used:
            result.append(node.val)
        else:
            used.add(node)
            stack.append(node)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
    return result