Binary Tree PreOrder traversal in java

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This is 2nd part of java binary tree tutorial.

In this post, we will see about PreOrder binary tree traversal in java.

PreOrder traversal:

In PreOrder traversal,each node is processed before either of its sub-trees.In simpler words,Visit each node before its children.
Steps for PreOrder traversal are:
  • Visit the node.
  • Traverse the left subtree in PreOrder.
  • Traverse the right subtree in PreOrder.

There can be two ways of implementing it

  • Recursive
  • Iterative

Recursive solution:
Recursive solution is very straight forward.Below diagram will make you understand recursion better.

Code for recursion will be:

Iterative solution:
For recursion, we use implicit stack. So here to convert recursive solution to iterative, we will use explicit stack.
Steps for iterative solution:

  1. Create empty stack and pust root node to it.
  2. Do the following when stack is not empty
    • Pop a node from stack and print it
    • Push right child of popped node to stack
    • Push left child of popped node to stack

We are pushing right child first, so it will be processed after left subtree as Stack is LIFO.

Lets say your binary tree is :

Lets create java program for PreOrder traversal:

Run above program and you will get following output:

Using Recursive solution:
40 20 10 30 60 50 70
Using Iterative solution:
40 20 10 30 60 50 70

Java Binary tree tutorial:

Please go through java interview programs for more such programs.

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