LeetCode-103. Binary Tree Zigzag Level Order Traversal

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],

return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]

采用同LeetCode-102中方法一相似的思路，不同之处在于要判断当前的层数来决定最后是否将一层的所有数值反转。

class Solution {    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {        List<List<Integer>> res=new ArrayList<List<Integer>>();        if(root==null)            return res;        LinkedList<TreeNode> q=new LinkedList<TreeNode>();        q.add(root);        int level=1;        while(!q.isEmpty())        {            int levelNum=q.size();            List<Integer> tempList=new ArrayList<Integer>();            for(int i=0;i<levelNum;++i)            {                if(q.peek().left!=null)                    q.add(q.peek().left);                if(q.peek().right!=null)                    q.add(q.peek().right);                tempList.add(q.poll().val);            }            if(level%2==0)                Collections.reverse(tempList);            res.add(tempList);            level++;        }        return res;    }}