LeetCode - Binary Tree Zigzag Level Order Traversal

题目：

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

思路：

采用Binary Tree Level Order Traversal的方法，然后对第二行、第四行进行reverse。

package treetraversal;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class BinaryTreeZigzagLevelOrderTraversal {

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        //List<Integer> record = new ArrayList<Integer>();
        levelOrderTraversal(res, root, 0);
        for (int i = 0; i < res.size(); ++i) {
            if (i % 2 == 1) {
                Collections.reverse(res.get(i));
            }
        }
        return res;
    }

    private void levelOrderTraversal(List<List<Integer>> res, TreeNode root, int k) {
        if (root == null) return;
        if (res.size() < k + 1) {
            List<Integer> record = new ArrayList<Integer>();
            record.add(root.val);
            res.add(record);
        } else {
            res.get(k).add(root.val);
        }
        levelOrderTraversal(res, root.left, k + 1);
        levelOrderTraversal(res, root.right, k + 1);
    }
    
}