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PostOrderTraversal.java
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PostOrderTraversal.java
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package com.anudev.ds.trees;
import java.util.Stack;
/**
* In pre-order and in-order after popping we do no need to visit the
* same vertex again. But in post-order we need to process is node twice.
* After processing the left subtree we will visit the current node and
* after processing the right subtree we visit the current node. But we
* need to process the current node during second visit.
*/
public class PostOrderTraversal {
public static void postOrderTraversal(Node node) {
System.out.print("Recursive Post order traversal: \t");
recursivePostOrderTraversal(node);
System.out.println();
System.out.print("Non Recursive Post order traversal: ");
nonRecursivePostOrderTraversal(node);
System.out.println();
}
private static void recursivePostOrderTraversal(Node node) {
if (node != null) {
recursivePostOrderTraversal(node.getLeftNode());
recursivePostOrderTraversal(node.getRightNode());
System.out.print(node.getValue() + ",");
}
}
private static void nonRecursivePostOrderTraversal(Node node) {
if (node == null) {
return;
}
Stack<Node> stack = new Stack<>();
while (true) {
// try to push nodes which are there in left subtree
if (node != null) {
// push the node in the stack
stack.push(node);
node = node.getLeftNode();
} else {
// check if stack is empty
if (stack.isEmpty()) {
break;
}
// check of the stack top element's right subtree is null
if (stack.peek().getRightNode() == null) {
node = stack.pop();
System.out.print(node.getValue() + ",");
// check if the current node is right side of
// the node in the stack
while (!stack.isEmpty() && node == stack.peek().getRightNode()) {
node = stack.pop();
System.out.print(node.getValue() + ",");
}
}
// if stack is empty
if (stack.isEmpty()) {
node = null;
} else {
node = stack.peek().getRightNode();
}
}
}
}
}