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| 1 | +package com.leetcode.trees; |
| 2 | + |
| 3 | +import java.util.*; |
| 4 | + |
| 5 | +import static org.junit.jupiter.api.Assertions.assertEquals; |
| 6 | + |
| 7 | +/** |
| 8 | + * Level: Medium |
| 9 | + * Link: https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/ |
| 10 | + * Description: |
| 11 | + * Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then |
| 12 | + * right to left for the next level and alternate between). |
| 13 | + * |
| 14 | + * For example: |
| 15 | + * Given binary tree [3,9,20,null,null,15,7], |
| 16 | + * 3 |
| 17 | + * / \ |
| 18 | + * 9 20 |
| 19 | + * / \ |
| 20 | + * 15 7 |
| 21 | + * return its zigzag level order traversal as: |
| 22 | + * [ |
| 23 | + * [3], |
| 24 | + * [20,9], |
| 25 | + * [15,7] |
| 26 | + * ] |
| 27 | + * |
| 28 | + * @author rampatra |
| 29 | + * @since 2019年08月11日 |
| 30 | + */ |
| 31 | +public class BinaryTreeZigZagLevelOrderTraversal { |
| 32 | + |
| 33 | + /** |
| 34 | + * Time Complexity: |
| 35 | + * Space Complexity: |
| 36 | + * Runtime: <a href="https://leetcode.com/submissions/detail/250830618/">1 ms</a>. |
| 37 | + * |
| 38 | + * @param root |
| 39 | + * @return |
| 40 | + */ |
| 41 | + public static List<List<Integer>> zigzagLevelOrder(TreeNode root) { |
| 42 | + |
| 43 | + int levelNo = 0; |
| 44 | + LinkedList<Integer> currLevel = new LinkedList<>(); |
| 45 | + List<List<Integer>> levelOrderTraversal = new LinkedList<>(); |
| 46 | + |
| 47 | + if (root == null) { |
| 48 | + return levelOrderTraversal; |
| 49 | + } |
| 50 | + |
| 51 | + Queue<TreeNode> queue = new LinkedList<>(); |
| 52 | + queue.add(root); |
| 53 | + queue.add(null); |
| 54 | + |
| 55 | + while (!queue.isEmpty()) { |
| 56 | + |
| 57 | + TreeNode treeNode = queue.poll(); |
| 58 | + |
| 59 | + if (treeNode == null) { |
| 60 | + levelOrderTraversal.add(currLevel); |
| 61 | + currLevel = new LinkedList<>(); |
| 62 | + levelNo++; |
| 63 | + |
| 64 | + if (queue.size() > 0) { |
| 65 | + queue.add(null); |
| 66 | + } |
| 67 | + } else { |
| 68 | + if (levelNo % 2 == 0) { |
| 69 | + currLevel.add(treeNode.val); |
| 70 | + } else { |
| 71 | + currLevel.add(0, treeNode.val); |
| 72 | + } |
| 73 | + if (treeNode.left != null) queue.add(treeNode.left); |
| 74 | + if (treeNode.right != null) queue.add(treeNode.right); |
| 75 | + } |
| 76 | + } |
| 77 | + |
| 78 | + return levelOrderTraversal; |
| 79 | + } |
| 80 | + |
| 81 | + public static void main(String[] args) { |
| 82 | + /* |
| 83 | + Binary Tree |
| 84 | + |
| 85 | + 1 |
| 86 | + / \ |
| 87 | + 2 3 |
| 88 | + / \ |
| 89 | + 4 5 |
| 90 | + */ |
| 91 | + TreeNode tree = new TreeNode(1); |
| 92 | + tree.left = new TreeNode(2); |
| 93 | + tree.right = new TreeNode(3); |
| 94 | + tree.left.left = new TreeNode(4); |
| 95 | + tree.left.right = new TreeNode(5); |
| 96 | + |
| 97 | + assertEquals("[[1], [3, 2], [4, 5]]", zigzagLevelOrder(tree).toString()); |
| 98 | + assertEquals("[]", zigzagLevelOrder(null).toString()); |
| 99 | + } |
| 100 | +} |
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