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]
]

import utils.TreeNode;

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

public class  {
  public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
    List<List<Integer>> ans = new ArrayList<>();
    helper(ans, root, 0);
    return ans;

  }

  public void helper(List<List<Integer>> ans, TreeNode root, int level) {
    if (root == null) return ;
    if (ans.size() == level) {
      ans.add(new ArrayList<>());
    }
    List<Integer> t = ans.get(level);
    if (level % 2 == 0) {
      t.add(root.val);
    }
    else
      t.add(t.size()-1, root.val);

    helper(ans, root.left, level+1);
    helper(ans, root.right, level+1);

  }

}

leetcode 103 binary tree zigzag level order traversal

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