For example:
Given binary tree {3,9,20,#,#,15,7},
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]
Binary Tree Level Order Traversal的变形题目,我们只要在上一题的基础上维护一个变量level,来判断是偶数层还是奇数层,如果是奇数层我们就正序添加,如果是偶数层就倒叙添加。我们可以用链表的addFirst() 方法。代码如下:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public List<List<Integer>> zigzagLevelOrder(TreeNode root) { List<List<Integer>> result = new ArrayList<List<Integer>>(); LinkedList<Integer> list = new LinkedList<Integer>(); Queue<TreeNode> queue = new LinkedList<TreeNode>(); if(root == null) return result; queue.offer(root); int count = 1, level = 1, helper = 0; while(!queue.isEmpty()) { TreeNode cur = queue.poll(); count --; if(level % 2 == 1) { list.add(cur.val); } else { list.addFirst(cur.val); } if(cur.left != null) { queue.offer(cur.left); helper ++; } if(cur.right != null) { queue.offer(cur.right); helper ++; } if(count == 0) { result.add(new LinkedList<Integer>(list)); count = helper; helper = 0; level ++; list.clear(); } } return result; } }