[Golang][Leetcode][BinaryTree]刷題系列-144-Preorder Traversal

Steven included in Leetcode Golang

2021-08-24 312 words 2 minutes

/leetcode144_binarytree/golang_leetcode.png

Contents

144. Binary Tree Preorder Traversal

Level : Easy

原題連結 : Click

題目 :

Given the root of a binary tree, return the preorder traversal of its nodes' values.

Example :

Note

Example 1:

Input: root = [1,null,2,3] Output: [1,2,3]

Example 2:

Input: root = [] Output: []

Example 3:

Input: root = [1] Output: [1]

Example 4:

Input: root = [1,2] Output: [1,2]

Example 5:

Input: root = [1,null,2] Output: [1,2]

解題思路 :

這題需要我們做preorder traversal
類似的題目還有inorder & postorder
若要分辨之間的差異需要先將binary tree的定義釐清，binary tree是由一個root node 加上 left child tree(左子樹) 加上 right child tree(右子樹)組成
所以根據visit的順序可以分成preorder , inorder , postorder
1. preorder visit順序 : root -> left -> right
2. inorder visit順序 : left -> root -> right
3. postorder順序 : left -> right -> root

所以在這題實現preorder traversal我們須先將visit root 再 visit 左子樹再 visit 右子樹
做binary tree traversal可以用recursive or Iterative(Stack)

Recursive解法

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

Runtime: 0 ms, faster than 100.00% of Go online submissions for Binary Tree Preorder Traversal.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
func preorderTraversal(root *TreeNode) []int {
    
    res := make([]int,0)
    var preorder func(node *TreeNode)
    preorder = func(node *TreeNode) {
        if node == nil {
            return
        }
        res = append(res, node.Val)
        preorder(node.Left)
        preorder(node.Right)
        
    }
    
    preorder(root)
    
    return res
}

Iterative(Stack)解法

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
func preorderTraversal(root *TreeNode) []int {
    
    var res []int
    
    var stack []*TreeNode
    
    if root == nil {
        return res
    }
    
    stack = append(stack, root)

    for len(stack) != 0 {
        
        visited := stack[len(stack)-1]
        stack = stack[:len(stack)-1]
        res = append(res,visited.Val)
        
        if visited.Right != nil {
            stack = append(stack, visited.Right)
        }
        
        if visited.Left != nil {
            stack = append(stack, visited.Left)
        }
        
    }
    
    return res

}

最後祝福努力認真的各位 “All your dream of are hidden in your daily life” 我們峰頂見!!!