106.buildTree.py

# Given two integer arrays inorder and postorder where inorder is the inorder
# traversal of a binary tree and postorder is the postorder traversal of the same
# tree, construct and return the binary tree.
#
#
#  Example 1:
#
#
# Input: inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
# Output: [3,9,20,null,null,15,7]
#
#
#  Example 2:
#
#
# Input: inorder = [-1], postorder = [-1]
# Output: [-1]
#
#
#
#  Constraints:
#
#
#  1 <= inorder.length <= 3000
#  postorder.length == inorder.length
#  -3000 <= inorder[i], postorder[i] <= 3000
#  inorder and postorder consist of unique values.
#  Each value of postorder also appears in inorder.
#  inorder is guaranteed to be the inorder traversal of the tree.
#  postorder is guaranteed to be the postorder traversal of the tree.
#
#
#  Related Topics Array Hash Table Divide and Conquer Tree Binary Tree 👍 7735 ?
# ? 122
from typing import List, Optional


# leetcode submit region begin(Prohibit modification and deletion)
# Definition for a binary tree node.
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class Solution:
    def buildTree(self, inorder: List[int], postorder: List[int]) -> TreeNode:
        # 第一步: 特殊情况讨论: 树为空. (递归终止条件)
        if not postorder:
            return None

        # 第二步: 后序遍历的最后一个就是当前的中间节点.
        root_val = postorder[-1]
        root = TreeNode(root_val)

        # 第三步: 找切割点.
        separator_idx = inorder.index(root_val)

        # 第四步: 切割inorder数组. 得到inorder数组的左,右半边.
        inorder_left = inorder[:separator_idx]
        inorder_right = inorder[separator_idx + 1:]

        # 第五步: 切割postorder数组. 得到postorder数组的左,右半边.
        # ⭐️ 重点1: 中序数组大小一定跟后序数组大小是相同的.
        postorder_left = postorder[:len(inorder_left)]
        postorder_right = postorder[len(inorder_left): len(postorder) - 1]

        # 第六步: 递归
        root.left = self.buildTree(inorder_left, postorder_left)
        root.right = self.buildTree(inorder_right, postorder_right)
         # 第七步: 返回答案
        return root


# leetcode submit region end(Prohibit modification and deletion)