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Added Zigzag Traversal of a Binary Tree (#3811)
* Added Zigzag Traversal of a Binary Tree * fixed file name Co-authored-by: Albina Gimaletdinova <gimaletdinovaalbina@gmail.com>
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package com.thealgorithms.datastructures.trees;
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import java.util.*;
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/**
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* Given a binary tree.
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* This code returns the zigzag level order traversal of its nodes' values.
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* Binary tree:
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* 7
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* / \
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* 6 3
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* / \ / \
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* 2 4 10 19
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* Zigzag traversal:
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* [[7], [3, 6], [2, 4, 10, 19]]
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* <p>
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* This solution implements the breadth-first search (BFS) algorithm using a queue.
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* 1. The algorithm starts with a root node. This node is added to a queue.
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* 2. While the queue is not empty:
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* - each time we enter the while-loop we get queue size. Queue size refers to the number of nodes at the current level.
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* - we traverse all the level nodes in 2 ways: from left to right OR from right to left
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* (this state is stored on `prevLevelFromLeftToRight` variable)
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* - if the current node has children we add them to a queue
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* - add level with nodes to a result.
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* <p>
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* Complexities:
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* O(N) - time, where N is the number of nodes in a binary tree
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* O(N) - space, where N is the number of nodes in a binary tree
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*
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* @author Albina Gimaletdinova on 11/01/2023
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*/
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public class ZigzagTraversal {
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public static List<List<Integer>> traverse(BinaryTree.Node root) {
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if (root == null) {
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return new ArrayList<>();
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}
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List<List<Integer>> result = new ArrayList<>();
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// create a queue
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Deque<BinaryTree.Node> q = new ArrayDeque<>();
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q.offer(root);
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// start with writing nodes from left to right
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boolean prevLevelFromLeftToRight = false;
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while (!q.isEmpty()) {
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int nodesOnLevel = q.size();
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List<Integer> level = new LinkedList<>();
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// traverse all the level nodes
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for (int i = 0; i < nodesOnLevel; i++) {
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BinaryTree.Node node = q.poll();
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if (prevLevelFromLeftToRight) {
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level.add(0, node.data);
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} else {
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level.add(node.data);
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}
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if (node.left != null) {
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q.offer(node.left);
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}
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if (node.right != null) {
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q.offer(node.right);
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}
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}
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// the next level node traversal will be from the other side
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prevLevelFromLeftToRight = !prevLevelFromLeftToRight;
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result.add(level);
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}
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return result;
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}
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}
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@ -0,0 +1,30 @@
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package com.thealgorithms.datastructures.trees;
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import org.junit.jupiter.api.Test;
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import java.util.Collections;
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import java.util.List;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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/**
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* @author Albina Gimaletdinova on 11/01/2023
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*/
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public class ZigzagTraversalTest {
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@Test
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public void testRootNull() {
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assertEquals(Collections.emptyList(), ZigzagTraversal.traverse(null));
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}
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@Test
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public void testSingleNodeTree() {
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final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{50});
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assertEquals(List.of(List.of(50)), ZigzagTraversal.traverse(root));
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}
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@Test
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public void testZigzagTraversal() {
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final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{7, 6, 14, 2, 80, 100});
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assertEquals(List.of(List.of(7), List.of(14, 6), List.of(2, 80, 100)), ZigzagTraversal.traverse(root));
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}
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}
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