LeetCode #106 Construct Binary Tree from Inorder and Postorder Traversal

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Problem

Given inorder and postorder traversal of a tree, construct the binary tree.

Example

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inorder = [9,3,15,20,7]
postorder = [9,15,7,20,3]

Return the following binary tree:

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5
  3
 / \
9  20
  /  \
 15   7

Solution

Method: Divide and Conquer

Time Complexity:

Space Complexity:

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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 buildTree(self, inorder, postorder):
        """
        :type inorder: List[int]
        :type postorder: List[int]
        :rtype: TreeNode
        """
        if not inorder:
            return None
        elif len(inorder) == 1:
            return TreeNode(inorder[0])
        else:
            partition = inorder.index(postorder[-1])
            root = TreeNode(postorder[-1])
            root.left = self.buildTree(inorder[:partition], postorder[:partition])
            root.right = self.buildTree(inorder[1 + partition:], postorder[partition: -1])
            
            return root

or

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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:
    # @param inorder, a list of integers
    # @param postorder, a list of integers
    # @return a tree node
    # 12:00
    def buildTree(self, inorder, postorder):
        if not inorder or not postorder:
            return None
        
        root = TreeNode(postorder.pop())
        inorderIndex = inorder.index(root.val)

        root.right = self.buildTree(inorder[inorderIndex+1:], postorder)
        root.left = self.buildTree(inorder[:inorderIndex], postorder)

        return root