145_Binary Tree Postorder Traversal

145. Binary Tree Postorder Traversal

Level: hard

Tag: tree, stack

Question

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

Example 1

Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [3,2,1]

Solution 1: recursive

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

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ret = []
        self.dfs(root, ret)
        return ret

    def dfs(self, root, ret):
        if root:
            self.dfs(root.left, ret)
            self.dfs(root.right, ret)
            ret.append(root.val)

Solution 2: iteratively (my solution, easy to understand)

use stack to store node value, right child, and left child repeatedly. When node value is popped out, add to result, otherwise repeat.

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

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack = [root]
        ret = []
        while stack:
            node = stack.pop()
            if type(node) == int:
                ret.append(node)
            elif node:
                stack.append(node.val)
                stack.append(node.right)
                stack.append(node.left)
        return ret

Another version, popular online

return node value -> right child -> left child repeatedly, and then reverse the result.

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

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []
        stack = [root]
        ret = []
        while stack:
            node = stack.pop()
            ret.append(node.val)
            if node.left:
                stack.append(node.left)
            if node.right:
                stack.append(node.right)
        return ret[::-1]

Another version, hard to understand

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

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []
        stack = [root]
        ret = []
        pre = None
        while stack:
            cur = stack[-1]
            if pre is None or pre.left == cur or pre.right == cur:
                if cur.left:
                    stack.append(cur.left)
                elif cur.right:
                    stack.append(cur.right)
            elif pre == cur.left and cur.right:
                stack.append(cur.right)
            else:
                ret.append(cur.val)
                stack.pop()
            pre = cur
        return ret