added Construct Binary Tree Given pre&inorder traversals (medium)

Author: minhyeong-joe

Medium/ConstructBinaryTree/README.md

@@ -0,0 +1,26 @@
+# Construct Binary Tree from Preorder and Inorder Traversal
+
+[Leetcode Link](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/)
+
+## Problem:
+
+Given preorder and inorder traversal of a tree, construct the binary tree.
+
+## Example:
+
+```
+Given:
+preorder = [3,9,20,15,7]
+inorder = [9,3,15,20,7]
+
+Returns following binary tree:
+ 3
+ / \
+ 9 20
+ / \
+ 15 7
+```
+
+## Note:
+
+You may assume that duplicates do not exist in the tree.

Medium/ConstructBinaryTree/solution.py

@@ -0,0 +1,53 @@
+from typing import List
+
+# 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
+
+ def __str__(self):
+ tree = list()
+ queue = list()
+ queue.append(self)
+ while queue and not all(node is None for node in queue):
+ currentNode = queue.pop(0)
+ if currentNode is None:
+ tree.append(None)
+ else:
+ tree.append(currentNode.val)
+ queue.append(currentNode.left)
+ queue.append(currentNode.right)
+ return str(tree)
+
+
+class Solution:
+ def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
+ return self.recursiveBuild(0, 0, len(inorder), preorder, inorder)
+
+ def recursiveBuild(self, preorderIndex: int, inorderStart: int, inorderEnd: int, preorder: List[int], inorder: List[int]) -> TreeNode:
+ if inorderStart >= inorderEnd or preorderIndex >= len(preorder):
+ return None
+ node = TreeNode(preorder[preorderIndex])
+ # find index of this node value at inorder
+ inorderIndex = inorder.index(preorder[preorderIndex])
+ # left node of current node is always next element in preorder (also pass in left subarray range)
+ node.left = self.recursiveBuild(
+ preorderIndex+1, inorderStart, inorderIndex, preorder, inorder)
+ # right node of current node is current node + num of all left nodes + 1
+ numLeftNodes = inorderIndex - inorderStart
+ node.right = self.recursiveBuild(
+ preorderIndex+numLeftNodes+1, inorderIndex+1, inorderEnd, preorder, inorder)
+ return node
+
+
+sol = Solution()
+preorder = [3, 9, 20, 15, 7]
+inorder = [9, 3, 15, 20, 7]
+
+root = sol.buildTree(preorder, inorder)
+
+print(root)

README.md

@@ -83,6 +83,7 @@ Languages used: Java and Python
  - [Divide Array is Sets of K Consecutive Numbers](Medium/DivideArrayInKConsecNums)
  - [Perfect Squares](Medium/PerfectSquares)
  - [Score After Flipping Matrix](Medium/ScoreAfterFlippingMatrix)
+ - [Construct Binary Tree from Preorder and Inorder Traversal](Medium/ConstructBinaryTree)
 - Hard
  - [Maximum Score Words Formed by Letters](Hard/MaximumScoreWords)
  - [Reducing Dishes](Hard/ReducingDishes)