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],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

一刷
题解：用DFS，在子函数内传入level信息，并通过level的奇偶判断加在list首部还是尾部（所以list为linkedlist)

/**
 * 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> zigzagLevelOrder(TreeNode root) {
        List> res = new ArrayList<>();
        travel(root, res, 0);
        return res;
    }
    
    private void travel(TreeNode curr, List> sol, int level){
        if(curr == null) return;
        if(sol.size()<=level){
            List newLevel = new LinkedList<>();
            sol.add(newLevel);
        }
        List collection = sol.get(level);
        if(level%2 == 0) collection.add(curr.val);//left to right
        else collection.add(0, curr.val);//rigth to left
        
        travel(curr.left, sol, level+1);
        travel(curr.right, sol, level+1);
        
    }
}