Golang Program to Perform the preorder tree traversal

In Go programming language, pre-order traversal is a tree traversal technique in which the root node is visited first, then the left subtree, and finally the right subtree. A recursive function is used that calls itself on the root node, the left child node, the right child node, and finally the left child node again. When the node is nil, the recursion's basic case, occurs. We will execute pre-order traversal in this program using two methods- the TreeNode struct and the stacks.

Method 1: Using TreeNode struct

This method builds a tree node structure With a Value field, a pointer to the Left child node, and a pointer to the Right child node. The pre_order_traversal function uses a pointer to the root node to recursively visit the root node, the left subtree, and then the right subtree in the tree in pre-order. The pre_order_traversal method is called by the main function to print the tree's pre-order traversal after creating a sample tree.

Algorithm

• Step 1 − Create a package main and declare fmt(format package) package in the program where main produces executable codes and fmt helps in formatting input and output.

• Step 2 − Create a TreeNode struct with value of type int, left_val and right_val of type TreeNode.

• Step 3 − Create a function pre_order_traversal and in that particular function return whether the current node is null. Print the node's value at the moment.

• Step 4 − In the next step, call pre_order_traversal with the left child of the current node as the parameter to traverse the left subtree.

• Step 5 − Call pre_order_traversal with the right child of the current node as the parameter to navigate the correct subtree.

• Step 6 − For every child node, repeat steps 1-4 until all nodes have been visited.

• Step 7 − This algorithm visits the root node first, then the left subtree, and ultimately the right subtree as it traverses the tree via recursion. When a branch of the tree comes to an end or when all nodes have been visited, the recursion comes to an end.

Example

In this example we will use TreeNode struct to implement pre-order traversal. Let’s have a look at the code.

package main
import "fmt"

// Define a tree node structure
type TreeNode struct {
   Value     int
   Left_val  *TreeNode
   Right_val *TreeNode
}

// Function to traverse the tree in pre-order
func pre_order_traversal(root *TreeNode) {
   if root == nil {
      return
   }
   fmt.Print(root.Value, " ")
   pre_order_traversal(root.Left_val)
   pre_order_traversal(root.Right_val)
}

func main() {
   // Create a tree
   root := &TreeNode{Value: 10}
   root.Left_val = &TreeNode{Value: 20}
   root.Right_val = &TreeNode{Value: 30}
   root.Left_val.Left_val = &TreeNode{Value: 40}
   root.Left_val.Right_val = &TreeNode{Value: 50}
   
   // Call the pre-order traversal function
   fmt.Print("Pre-order traversal: ")
   pre_order_traversal(root)
   fmt.Println()
}

Output

Pre-order traversal: 10 20 40 50 30

Method 2: Using Stacks

This method employs a stack to keep track of the nodes that need to be visited. The method repeatedly removes a node from the stack, prints its value, and pushes its right and left children into the stack in that sequence. The stack starts with the root node. This results in a pre-order traversal of the tree, where each node's left child is examined before its right child.

Algorithm

• Step 1 − Create a package main and declare fmt(format package) package in the program where main produces executable codes and fmt helps in formatting input and output.

• Step 2 − Create a TreeNode struct with value of type int, left_val and right_val of type TreeNode.

• Step 3 − Create a function pre_order_traversal and in that function return if the root node is null.

• Step 4 − In the next step, create a stack from the root node up.

• Step 5 − Run a loop by checking if the length of the stack is greater than 0 and repeat the following actions as long as the stack is still full −

  a. Remove the node from the stack's top and assign it to a variable called curr.

  b. Print the current value.

• Step 6 − In the main function the function will be called and the print statement is executed using fmt.Println() function where ln means new line.

Example

In this example we will use stacks to show pre-order traversal. Let’s see the execution through the code.

package main
import (
   "fmt"
)

// Define a tree node structure
type TreeNode struct {
   Value     int
   Left_val  *TreeNode
   Right_val *TreeNode
}

// Traverse the tree in pre-order using a stack
func pre_order_traversal(root *TreeNode) {
   if root == nil {
      return
   }
   stack := []*TreeNode{root}
   for len(stack) > 0 {
      curr := stack[len(stack)-1]
      stack = stack[:len(stack)-1]
      fmt.Print(curr.Value, " ")
      if curr.Right_val != nil {
         stack = append(stack, curr.Right_val)
      }
      if curr.Left_val != nil {
         stack = append(stack, curr.Left_val)
      }
   }
}

func main() {
   // Create a tree
   root := &TreeNode{Value: 10}
   root.Left_val = &TreeNode{Value: 20}
   root.Right_val = &TreeNode{Value: 30}
   root.Left_val.Left_val = &TreeNode{Value: 40}
   root.Left_val.Right_val = &TreeNode{Value: 50}
   
   // Call the pre-order traversal function
   fmt.Print("Pre-order traversal: ")
   pre_order_traversal(root)
   fmt.Println()
}

Output

Pre-order traversal: 10 20 40 50 30

Conclusion

We executed the above program using two examples. In the first example we used the TreeNode struct and in the second example we used stacks on TreeNode struct to show pre-order traversal.