[Leetcode] 0105. Construct Binary Tree from Preorder and Inorder Traversal

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Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.

 

Example 1:

Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
Output: [3,9,20,null,null,15,7]

Example 2:

Input: preorder = [-1], inorder = [-1]
Output: [-1]

 

Constraints:

  • 1 <= preorder.length <= 3000
  • inorder.length == preorder.length
  • -3000 <= preorder[i], inorder[i] <= 3000
  • preorder and inorder consist of unique values.
  • Each value of inorder also appears in preorder.
  • preorder is guaranteed to be the preorder traversal of the tree.
  • inorder is guaranteed to be the inorder traversal of the tree.
 

內容目錄

Python

				
					# time complexity: O(n)
# space complexity: O(n)
from typing import List, Optional



class Solution:
    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:

        def dfs(left: int, right: int):
            nonlocal preorderIndex
            if left > right:
                return None
            rootValue = preorder[preorderIndex]
            root = TreeNode(rootValue)
            preorderIndex += 1
            root.left = dfs(left, inorderMap[rootValue] - 1)
            root.right = dfs(inorderMap[rootValue] + 1, right)
            return root
        
        preorderIndex = 0
        inorderMap = {}
        for i, value in enumerate(inorder):
            inorderMap[value] = i

        return dfs(0, len(inorder) - 1)


preorder = [3, 9, 20, 15, 7]
inorder = [9, 3, 15, 20, 7]
print(Solution().buildTree(preorder, inorder))

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