103 Binary Tree Zigzag Level Order Traversal

Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();

        if (root == null){
            return result;
        }

        int index = 0;
        boolean flag = false;
        helper(root, index, result);
        return result;
    }

    private void helper(TreeNode root, int index, List<List<Integer>> result){
        if (root == null){
            return;
        }

        if (index >= result.size()){
            result.add(new ArrayList<Integer>());
        }

        if (index % 2 == 0){
            result.get(index).add(root.val);
        }else{
            result.get(index).add(0, root.val); // 0 means add first place.
        }

        helper(root.left, index+1, result);
        helper(root.right, index+1, result);
    }

}
//TC: O(n)
//SC: O(n)