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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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