106 - Construct Binary Tree from Inorder and Postorder Traversal

106. Construct Binary Tree from Inorder and Postorder Traversal

Problem Statement

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.

Solutions

Go

package main

// Definition for a binary tree node.
type TreeNode struct {
	Val   int
	Left  *TreeNode
	Right *TreeNode
}

func buildTree(inorder []int, postorder []int) *TreeNode {
	mapInOrder := make(map[int]int)

	for i, v := range inorder {
		mapInOrder[v] = i
	}

	var helper func(int, int) *TreeNode
	helper = func(left, right int) *TreeNode {
		if left > right {
			return nil
		}

		pop := postorder[len(postorder)-1]
		postorder = postorder[:len(postorder)-1]

		root := &TreeNode{Val: pop}
		mid := mapInOrder[pop]
		root.Right = helper(mid+1, right)
		root.Left = helper(left, mid-1)

		return root
	}

	return helper(0, len(inorder)-1)
}

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