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Oct 5, 2018
Merged

add tree traversal in golang #406

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df1fa12
add the golang files
Sep 25, 2018
3923009
Merge branch 'master' into golang-tree-traversal
leios Oct 5, 2018
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90 changes: 90 additions & 0 deletions contents/tree_traversal/code/golang/treetraversal.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,90 @@
package main

import "fmt"

type node struct {
id int
children []*node
}

func dfsRecursive(n *node) {
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Consider returning an array of elements or taking a visitor function, rather than just printing all of them.

That goes for all of these functions, not just this one.

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I rather have a pointer since it is more memory efficient. I also find it way easier. Tbh i'm not really sure why an array would be better, since now i can build a native tree structure and don't have to build an array and do crazy offset math to figure out the children.

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@Liikt Yeah, that got shot down in other places too, since we're just demonstrating the algorithm, not trying to build reusable chunks of code. Disregard this one.

fmt.Println(n.id)
for _, child := range n.children {
dfsRecursive(child)
}
}

func dfsRecursivePostorder(n *node) {
for _, child := range n.children {
dfsRecursive(child)
}
fmt.Println(n.id)
}

func dfsRecursiveInorderBtree(n *node) {
switch len(n.children) {
case 2:
dfsRecursiveInorderBtree(n.children[0])
fmt.Println(n.id)
dfsRecursiveInorderBtree(n.children[1])
case 1:
dfsRecursiveInorderBtree(n.children[0])
fmt.Println(n.id)
case 0:
fmt.Println(n.id)
default:
fmt.Println("This is not a binary tree")
}
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This can be simplified a bit by running code if lengths are greater than a certain amount, rather than just if they're equal to it. I'm not confident enough with Golang to suggest an alternative without an interpreter handy, but in pseudocode:

if children.length > 2:
  throw "Not a binary tree"

if children.length > 1:
  inOrder(children[1])
printThis()
if children.length > 0:
  inOrder(children[0])

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I agree with you here but I prefer the switch just for readability. It can definitely be made shorter like that though.

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@Liikt Yup! Which is why I still approved. Both of these were ideas, not must-fixes.

}

func dfsStack(n *node) {
stack := []*node{n}

for len(stack) > 0 {
cur := stack[0]
stack = stack[1:]
fmt.Println(cur.id)
stack = append(cur.children, stack...)
}
}

func bfsQueue(n *node) {
queue := []*node{n}

for len(queue) > 0 {
cur := queue[0]
queue = queue[1:]
fmt.Println(cur.id)
queue = append(queue, cur.children...)
}
}

func createTree(numRow, numChild int) *node {
if numRow == 0 {
return &node{id: 0}
}

cur := new(node)
cur.id = numRow

for x := 0; x < numChild; x++ {
cur.children = append(cur.children, createTree(numRow-1, numChild))
}
return cur
}

func main() {
root := createTree(3, 3)
binTree := createTree(3, 2)

fmt.Println("DFS recursive:")
dfsRecursive(root)
fmt.Println("DFS post order recursive:")
dfsRecursivePostorder(root)
fmt.Println("DFS inorder binary tree:")
dfsRecursiveInorderBtree(binTree)
fmt.Println("DFS stack:")
dfsStack(root)
fmt.Println("BFS queue:")
bfsQueue(root)
}
14 changes: 14 additions & 0 deletions contents/tree_traversal/tree_traversal.md
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Expand Up @@ -32,6 +32,8 @@ This has not been implemented in your chosen language, so here is the Julia code
[import:3-27, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:1-5, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import:5-8, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}

Because of this, the most straightforward way to traverse the tree might be recursive. This naturally leads us to the Depth-First Search (DFS) method:
Expand Down Expand Up @@ -66,6 +68,8 @@ Because of this, the most straightforward way to traverse the tree might be recu
[import:31-35, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:7-10, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import:10-15, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}

At least to me, this makes a lot of sense. We fight recursion with recursion! First, we first output the node we are on and then we call `DFS_recursive(...)` on each of its children nodes. This method of tree traversal does what its name implies: it goes to the depths of the tree first before going through the rest of the branches. In this case, the ordering looks like:
Expand Down Expand Up @@ -108,6 +112,8 @@ Now, in this case the first element searched through is still the root of the tr
[import:37-41, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:12-15, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import:17-22, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}

<p>
Expand Down Expand Up @@ -145,6 +151,8 @@ In this case, the first node visited is at the bottom of the tree and moves up t
[import:43-62, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:17-31, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import:24-38, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}

<p>
Expand Down Expand Up @@ -192,6 +200,8 @@ In code, it looks like this:
[import:64-73, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:33-41, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import:40-49, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}

All this said, there are a few details about DFS that might not be idea, depending on the situation. For example, if we use DFS on an incredibly long tree, we will spend a lot of time going further and further down a single branch without searching the rest of the data structure. In addition, it is not the natural way humans would order a tree if asked to number all the nodes from top to bottom. I would argue a more natural traversal order would look something like this:
Expand Down Expand Up @@ -231,6 +241,8 @@ And this is exactly what Breadth-First Search (BFS) does! On top of that, it can
[import:65-74, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:43-51, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import:51-60, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}

## Example Code
Expand Down Expand Up @@ -272,6 +284,8 @@ The code snippets were taken from this [Scratch project](https://scratch.mit.edu
[import, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="go" %}
[import, lang:"golang"](code/golang/treetraversal.go)
{% endmethod %}


Expand Down