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Tree_BinaryTreePostOrderTraversal.java
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Tree_BinaryTreePostOrderTraversal.java
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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.Objects;
import java.util.Queue;
import java.util.Stack;
// https://practice.geeksforgeeks.org/problems/postorder-traversal/1/
// https://leetcode.com/problems/binary-tree-postorder-traversal/
// https://www.geeksforgeeks.org/tree-traversals-inorder-preorder-and-postorder/
// https://github.com/mission-peace/interview/blob/master/src/com/interview/tree/TreeTraversals.java
// https://www.youtube.com/watch?v=qT65HltK2uE (Best Video Explanation)
// https://www.youtube.com/watch?v=xLQKdq0Ffjg (BEST Video on non recursive with single stack)
public class Tree_BinaryTreePostOrderTraversal {
static class Tree {
void printPostOrderRecursive(Node root) {
if (root == null) return;
printPostOrderRecursive(root.left);
printPostOrderRecursive(root.right);
System.out.print(root.data + " ");
}
void printPostOrderRecursiveWithTwoStack(Node root) {
Stack<Node> stackA = new Stack<>();
Stack<Node> stackB = new Stack<>();
Node currNode = root;
stackA.push(currNode);
while (!stackA.isEmpty()) {
currNode = stackA.pop();
stackB.push(currNode);
if(Objects.nonNull(currNode.left)) stackA.push(currNode.left);
if(Objects.nonNull(currNode.right)) stackA.push(currNode.right);
}
print(stackB);
}
void print(Stack<Node> stack) {
if(stack == null) return;
StringBuilder sb = new StringBuilder();
while (!stack.isEmpty()) {
sb.append(stack.pop().data).append(" ");
}
System.out.print(sb);
sb.setLength(0);
}
// https://www.youtube.com/watch?v=xLQKdq0Ffjg (BEST Video on non recursive with single stack)
// https://github.com/mission-peace/interview/blob/master/src/com/interview/tree/TreeTraversals.java#L98
void printPostOrderRecursiveWithOneStack(Node root) {
StringBuilder sb = new StringBuilder();
Stack<Node> stack = new Stack<>();
Node currNode = root;
/* be on the loop when stack is not empty and currNode is not null */
while (currNode != null || !stack.isEmpty()) {
if (currNode != null) {
stack.push(currNode);
currNode = currNode.left;
}
else {
/* at this point, currNode is null but stack is not empty,
* so now we are gonna check does the node at the top of the stack has a right child or not*/
Node temp = stack.peek().right;
if(temp == null) {
/* at this point both the left and right child node is already visited or both are null
* so print the value */
temp = stack.pop();
sb.append(temp.data).append(" ");
/* now we check that temp is right child of top of the stack or not.
* if it is the right child then we are already visited both right child and left child,
* so print the value */
while (!stack.isEmpty() && temp == stack.peek().right) {
temp = stack.pop();
sb.append(temp.data).append(" ");
}
}
else {
/* it it almost similar as currNode = currNode.right,
* so here we actually go deeper in the right child */
currNode = temp;
}
}
}
System.out.println(sb);
sb.setLength(0);
}
}
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int t = Integer.parseInt(br.readLine());
while (t-- > 0) {
String s = br.readLine().trim();
Node root = buildTree(s);
Tree tree = new Tree();
System.out.println("Recursive : ");
tree.printPostOrderRecursive(root);
System.out.println();
System.out.println("Non recursive with two stack : ");
tree.printPostOrderRecursiveWithTwoStack(root);
System.out.println();
System.out.println("Non recursive with one stack : ");
tree.printPostOrderRecursiveWithOneStack(root);
System.out.println();
}
}
private static Node buildTree(String str) {
if (str.length()==0 || str.charAt(0)=='N') {
return null;
}
String[] ip = str.split("\\s+");
Node root = new Node(Integer.parseInt(ip[0]));
Queue<Node> queue = new LinkedList<>();
queue.add(root);
int i = 1;
while(!queue.isEmpty() && i < ip.length) {
Node currNode = queue.peek();
queue.remove();
String currData = ip[i];
if(!currData.equals("N")) {
currNode.left = new Node(Integer.parseInt(currData));
queue.add(currNode.left);
}
i++;
if(i >= ip.length) break;
currData = ip[i];
if(!currData.equals("N")) {
currNode.right = new Node(Integer.parseInt(currData));
queue.add(currNode.right);
}
i++;
}
return root;
}
static class Node {
int data;
Node left;
Node right;
Node(int data){
this.data = data;
left = right = null;
}
}
}