Lc 105. 從前序與中序遍歷序列構造二叉樹
根據一棵樹的前序遍歷與中序遍歷構造二叉樹。
注意:
你可以假設樹中沒有重複的元素。
例如,給出
前序遍歷 preorder = [3,9,20,15,7]
中序遍歷 inorder = [9,3,15,20,7]
遞歸求解,把中序遍歷的值和下標存入hash表中:
# 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: List[int], inorder: List[int]) -> TreeNode:
if not preorder or not inorder or len(preorder) == 0 or len(inorder)==0:
return
mapping = {}
for index in range(0,len(inorder)):
mapping[inorder[index]] = index
rootval = preorder[0]
root = TreeNode(rootval)
rootindex = mapping[rootval]
root.left = self.buildTree(preorder[1:1+rootindex],inorder[0:rootindex])
root.right = self.buildTree(preorder[1+rootindex:],inorder[rootindex+1:])
return root
Lc 106 從中序與後序遍歷序列構造二叉樹
根據一棵樹的中序遍歷與後序遍歷構造二叉樹。
注意:
你可以假設樹中沒有重複的元素。
例如,給出
中序遍歷 inorder = [9,3,15,20,7]
後序遍歷 postorder = [9,15,7,20,3]
# 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: List[int], postorder: List[int]) -> TreeNode:
if not inorder or not postorder or len(inorder) == 0 or len(postorder)==0:
return
mapping = {}
for index in range(0,len(inorder)):
mapping[inorder[index]] = index
rootval = postorder[-1]
root = TreeNode(rootval)
rootindex = mapping[rootval]
root.left = self.buildTree(inorder[0:rootindex],postorder[0:rootindex])
root.right = self.buildTree(inorder[rootindex+1:],postorder[rootindex:-1])
return root