[LeetCode]106 从中序与后序遍历序列构造二叉树

题目描述

根据一棵树的中序遍历与后序遍历构造二叉树。

注意:
你可以假设树中没有重复的元素。

例如，给出

中序遍历 inorder = [9,3,15,20,7]
后序遍历 postorder = [9,15,7,20,3]

返回如下的二叉树：

  3
 / \
9  20
  /  \
 15   7

代码

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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, preorder, inorder):
        """
        :type preorder: List[int]
        :type inorder: List[int]
        :rtype: TreeNode
        """
        if not preorder or not inorder:
            return 
        
        root = TreeNode(preorder[0])
        stack = [root]
        i = 0
        
        for node in preorder[1:]:
            parent = stack[-1]
            
            if parent.val != inorder[i]:
                parent.left = TreeNode(node)
                stack.append(parent.left)
            else:
                while stack and inorder[i] == stack[-1].val:
                    parent = stack.pop()
                    i += 1
                    
                parent.right = TreeNode(node)
                stack.append(parent.right)
                
        return root