LeetCode #889 Construct Binary Tree from Preorder and Postorder Traversal

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Problem

Return any binary tree that matches the given preorder and postorder traversals.

Values in the traversals pre and post are distinct positive integers.

Example

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Input: pre = [1,2,4,5,3,6,7], post = [4,5,2,6,7,3,1]
Output: [1,2,3,4,5,6,7]

Note

  • 1 <= pre.length == post.length <= 30
  • pre[] and post[] are both permutations of 1, 2, ..., pre.length.
  • It is guaranteed an answer exists. If there exists multiple answers, you can return any of them.

Solution

Method: Divide and Conquer

Time Complexity: O(n)

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 constructFromPrePost(self, pre, post):
        """
        :type pre: List[int]
        :type post: List[int]
        :rtype: TreeNode
        """
        if not pre and not post:
            return None
        elif len(pre) == 1:
            return TreeNode(pre[0])
        elif len(pre) == 2:
            ret = TreeNode(pre[0])
            ret.left = TreeNode(pre[1])
            return ret
        else:
            root = TreeNode(pre[0])
            partition = post.index(pre[1]) + 1

            root.left = self.constructFromPrePost(pre[1: 1 + partition], post[:partition])
            root.right = self.constructFromPrePost(pre[1 + partition:], post[partition: - 1])
            
            return root