[Golang][Leetcode][BinaryTree]刷題系列-105-Construct Binary Tree from Preorder and Inorder Traversal

2021-09-05 254 words 2 minutes

/leetcode105_binarytree/golang_leetcode.png

Contents

105. Construct Binary Tree from Preorder and Inorder Traversal

Level : Medium

原題連結 : Click

題目 :

Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.

Example :

Note

Example 1:

Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7] Output: [3,9,20,null,null,15,7]

Example 2:

Input: preorder = [-1], inorder = [-1] Output: [-1]

解題思路 :

這題的概念可以直接參考 leetcode106題解，一樣是重構binary tree，只是這題使用到的資源是preorder & inorder
這題只是將106題postorder數組該成preorder數組，從原本取得最後一個數作為root改成取第一個數作為root
然後recursive code裡，切分的slice range改變，若理解了106題，那這題就可以輕鬆完成!

Recursive解法

time complexity: O(n^2) , space complexity: O(n)

Runtime: 4 ms, faster than 91.79% of Go online submissions for Construct Binary Tree from Preorder and Inorder Traversal.

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/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func buildTree(preorder []int, inorder []int) *TreeNode {
    
    if len(preorder) == 0 || len(inorder) == 0 {
        return nil
    }
    
    mid := preorder[0]
    
    midIndex := findIndex(inorder, mid)
    
    node := &TreeNode {
        Val: mid,
        Left: buildTree(preorder[1:midIndex+1], inorder[:midIndex]),
        Right: buildTree(preorder[midIndex+1:], inorder[midIndex+1:]),
    }
    
    return node
}

func findIndex( arr []int, tar int) int{
    
    for i, v := range arr {
        if v == tar {
            return i
        }
    }
    
    return -1
}

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