init impl

Author: freemanzhang

src/linkedIn/BinaryTreeZigzagLevelOrderTraversal.java

@@ -1,88 +1,101 @@
-package linkedIn;
-
-import java.util.ArrayList;
-import java.util.LinkedList;
-import java.util.List;
-import java.util.Stack;
-
-import org.junit.Test;
-
-import utility.TreeNode;
-
-/**
- * 
- */
-
-public class BinaryTreeZigzagLevelOrderTraversal
-{
-	@Test
-	public void test()
-	{
-		TreeNode root = new TreeNode(3);
-		TreeNode node1 = new TreeNode(9);
-		TreeNode node2 = new TreeNode(20);
-		TreeNode node3 = new TreeNode(15);
-		TreeNode node4 = new TreeNode(7);
-		root.left = node1;
-		root.right = node2;
-		node2.left = node3;
-		node2.right = node4;
-		System.out.println( zigzagLevelOrder( root ) );
-	}
-	
-    public List<List<Integer>> zigzagLevelOrder( TreeNode root )
-    {
-    	List<List<Integer>> allLevels = new ArrayList<>();
-    	if ( root == null )
-    	{
-    		return allLevels;
-    	}
-    	
-    	Stack<TreeNode> oddLevelStack = new Stack<>();
-    	Stack<TreeNode> evenLevelStack = new Stack<>();
-    	oddLevelStack.push( root );
-    	
-    	while ( !oddLevelStack.isEmpty() 
-    			|| !evenLevelStack.isEmpty() )
-    	{
-    		List<Integer> oneLevel = new LinkedList<>();
-    		// odd layer, push right child first, left child last
-    		if ( !oddLevelStack.isEmpty() )
-    		{    			
-    			while ( !oddLevelStack.isEmpty() )
-    			{
-    				TreeNode stackTop = oddLevelStack.pop();
-    				oneLevel.add( stackTop.val );
-    				if ( stackTop.left != null )
-    				{
-    					evenLevelStack.push( stackTop.left );
-    				}
-    				if ( stackTop.right != null )
-    				{
-    					evenLevelStack.push( stackTop.right );
-    				}
-    			}
-    		}
-    		// even layer, push left child first, right child last
-    		else
-    		{
-    			while ( !evenLevelStack.isEmpty() )
-    			{
-    				TreeNode stackTop = evenLevelStack.pop();
-    				oneLevel.add( stackTop.val );
-    				if ( stackTop.right != null )
-    				{
-    					oddLevelStack.push( stackTop.right );
-    				}
-    				if ( stackTop.left != null )
-    				{
-    					oddLevelStack.push( stackTop.left );
-    				}
-    			}
-    		}
-    		allLevels.add( oneLevel );
-    	}
-    	return allLevels;
-    }
-
-}
+package linkedIn;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Stack;
+
+import org.junit.Test;
+
+import utility.TreeNode;
+
+/**
+Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
+
+For example:
+Given binary tree [3,9,20,null,null,15,7],
+    3
+   / \
+  9  20
+    /  \
+   15   7
+return its zigzag level order traversal as:
+[
+  [3],
+  [20,9],
+  [15,7]
+]
+ */
+
+public class BinaryTreeZigzagLevelOrderTraversal
+{
+	@Test
+	public void test()
+	{
+		TreeNode root = new TreeNode(3);
+		TreeNode node1 = new TreeNode(9);
+		TreeNode node2 = new TreeNode(20);
+		TreeNode node3 = new TreeNode(15);
+		TreeNode node4 = new TreeNode(7);
+		root.left = node1;
+		root.right = node2;
+		node2.left = node3;
+		node2.right = node4;
+		System.out.println( zigzagLevelOrder( root ) );
+	}
+	
+    public List<List<Integer>> zigzagLevelOrder( TreeNode root )
+    {
+    	List<List<Integer>> result = new ArrayList<>();
+    	if ( root == null )
+    	{
+    		return result;
+    	}
+    	
+    	Stack<TreeNode> currLayer = new Stack<>();
+    	currLayer.push( root );
+    	Stack<TreeNode> nextLayer = new Stack<>();
+    	List<Integer> layerOutput = new ArrayList<>();
+    	int direction = 0;
+    	
+    	while ( !currLayer.isEmpty() )
+    	{
+    		TreeNode node = currLayer.pop();
+    		layerOutput.add( node.val );
+    		if ( direction == 0 )
+    		{
+    			if ( node.left != null )
+    			{
+    				nextLayer.push( node.left );
+    			}
+    			if ( node.right != null )
+    			{
+    				nextLayer.push( node.right );
+    			}
+    		}
+    		else
+    		{
+    			if ( node.right != null )
+    			{
+    				nextLayer.push( node.right );
+    			}
+    			if ( node.left != null )
+    			{
+    				nextLayer.push( node.left );
+    			}
+    		}
+    		
+    		if ( currLayer.isEmpty() )
+    		{
+    			result.add( layerOutput );
+    			layerOutput = new ArrayList<>();
+    			currLayer = nextLayer;
+    			nextLayer = new Stack<>();
+    			
+    			direction ^= 1;
+    		}
+    	}
+    	return result;    	
+    }
+
+}