Solve the problem #103

Author: rayworks

src/main/java/org/sean/tree/AvgLevelResolver.java

@@ -5,6 +5,8 @@
 import java.util.LinkedList;
 import java.util.List;
 
+import static org.sean.utils.MathLib.log2;
+
 /***
  * 637. Average of Levels in Binary Tree
  *
@@ -17,10 +19,6 @@
  * Output: [3, 14.5, 11]
  */
 public class AvgLevelResolver {
-    public static int log2(int n) {
-        return (int) (Math.log(n) / Math.log(2));
-    }
-
     /***
      * @see {@link LevelTraversalSolution}
      */

src/main/java/org/sean/tree/LevelTraversalSolution.java

@@ -4,16 +4,15 @@
 import java.util.LinkedList;
 import java.util.List;
 
+import static org.sean.utils.MathLib.log2;
+
 /**
  * Created by Sean on 11/17/16.
  * <p/>
  * Leetcode : 107. Binary Tree Level Order Traversal II
  * https://leetcode.com/problems/binary-tree-level-order-traversal-ii/
  */
 public class LevelTraversalSolution {
-    public static int log2(int n) {
-        return (int) (Math.log(n) / Math.log(2));
-    }
 
     public List<List<Integer>> levelOrderBottom(TreeNode root) {
         List<List<Integer>> resultList = new LinkedList<List<Integer>>();

src/main/java/org/sean/tree/ZigzagTraversal.java

@@ -0,0 +1,60 @@
+package org.sean.tree;
+
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+
+import static org.sean.utils.MathLib.log2;
+
+/***
+ * 103. Binary Tree Zigzag Level Order Traversal
+ */
+public class ZigzagTraversal {
+
+    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
+        List<List<Integer>> resultList = new LinkedList<List<Integer>>();
+        if (root == null) return resultList;
+
+        if (root.left == null && root.right == null) {
+            List<Integer> list = Arrays.asList(root.val);
+            resultList.add(list);
+            return resultList;
+        }
+
+        LinkedList<TreeNode> queue = new LinkedList<>();
+        LinkedList<Integer> sequences = new LinkedList<>();
+
+        queue.add(root);
+        sequences.add(1);
+
+        while (!queue.isEmpty()) {
+            TreeNode node = queue.pop();
+            int val = node.val;
+            int count = sequences.pop();
+
+            int index = log2(count);
+            if (resultList.size() < 1 + index) {
+                List<Integer> list = new LinkedList<>();
+                list.add(val);
+
+                resultList.add(list);
+            } else {
+                // elements order varies for different level
+                if (index % 2 == 0) resultList.get(index).add(val);
+                else resultList.get(index).add(0, val);
+            }
+
+            if (node.left != null) {
+                queue.add(node.left);
+                sequences.add(2 * count);
+            }
+
+            if (node.right != null) {
+                queue.add(node.right);
+                sequences.add(2 * count + 1);
+            }
+        }
+
+        return resultList;
+    }
+}

src/main/java/org/sean/utils/MathLib.java

@@ -0,0 +1,7 @@
+package org.sean.utils;
+
+public class MathLib {
+    public static int log2(int n) {
+        return (int) (Math.log(n) / Math.log(2));
+    }
+}