103. Binary Tree Zigzag Level Order Traversal

Given a binary tree, return thezigzag level ordertraversal 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]
]

思路：LinkedLIst 加add(0,x)可以加在头上。交替true和false的话order = order ? false : true

public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if (root==null){
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        boolean order = true;
        int curL = 0;

        while(!queue.isEmpty()){
            List<Integer> temp = new ArrayList<Integer>();
            curL = queue.size();
            for (int i=0;i<curL;i++){
                TreeNode peek = queue.remove();
                if (order){
                    temp.add(peek.val);
                }else{
                    temp.add(0,peek.val);
                }

                if (peek.left!=null){
                    queue.add(peek.left);
                }
                if (peek.right!=null){
                    queue.add(peek.right);
                }
            }
            res.add(temp);
            order = order ? false : true;
        }
        return res;
    }
}