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binary tree zigzag level order traversal

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EASY/src/easy
- BinaryTreeZigzagLevelOrderTraversal.java

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`‎EASY/src/easy/BinaryTreeZigzagLevelOrderTraversal.java`

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	`1`	`+packageeasy;`
	`2`	`+`
	`3`	`+importjava.util.ArrayList;`
	`4`	`+importjava.util.Collections;`
	`5`	`+importjava.util.LinkedList;`
	`6`	`+importjava.util.List;`
	`7`	`+importjava.util.Queue;`
	`8`	`+`
	`9`	`+importclasses.TreeNode;`
	`10`	`+`
	`11`	`+/**`
	`12`	`+ * 103. Binary Tree Zigzag Level Order Traversal`
	`13`	`+ *`
	`14`	`+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).`
	`15`	`+`
	`16`	`+For example:`
	`17`	`+Given binary tree [3,9,20,null,null,15,7],`
	`18`	`+ 3`
	`19`	`+ / \`
	`20`	`+ 9 20`
	`21`	`+ / \`
	`22`	`+ 15 7`
	`23`	`+return its zigzag level order traversal as:`
	`24`	`+[`
	`25`	`+ [3],`
	`26`	`+ [20,9],`
	`27`	`+ [15,7]`
	`28`	`+]`
	`29`	`+*/`
	`30`	`+publicclassBinaryTreeZigzagLevelOrderTraversal {`
	`31`	`+publicList<List<Integer>>zigzagLevelOrder(TreeNoderoot) {`
	`32`	`+Queue<TreeNode>q =newLinkedList();`
	`33`	`+List<List<Integer>>levels =newArrayList();`
	`34`	`+if(root ==null)returnlevels;`
	`35`	`+q.offer(root);`
	`36`	`+booleanforward =true;`
	`37`	`+while(!q.isEmpty()){`
	`38`	`+intsize =q.size();`
	`39`	`+List<Integer>level =newArrayList();`
	`40`	`+for(inti =0;i <size;i++){`
	`41`	`+TreeNodecurr =q.poll();`
	`42`	`+level.add(curr.val);`
	`43`	`+if(curr.left !=null)q.offer(curr.left);`
	`44`	`+if(curr.right !=null)q.offer(curr.right);`
	`45`	`+ }`
	`46`	`+if(forward){`
	`47`	`+forward =false;`
	`48`	`+levels.add(level);`
	`49`	`+ }else {`
	`50`	`+Collections.reverse(level);`
	`51`	`+levels.add(level);`
	`52`	`+forward =true;`
	`53`	`+ }`
	`54`	`+ }`
	`55`	`+returnlevels;`
	`56`	`+ }`
	`57`	`+}`

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`‎EASY/src/easy/BinaryTreeZigzagLevelOrderTraversal.java`

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