Leetcode103-binaryTreeZigZagLevelOrderTraversal

Description

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).

Example

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]
]

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        
        traversal(root, res, 0);
        
        return res;
    }
    public void traversal(TreeNode node, List<List<Integer>> res, int level){
        if (res.size() <= level) res.add(new ArrayList<>());
        
        if (level % 2 == 0) res.get(level).add(node.val);
        else res.get(level).add(0, node.val);
        
        if (node.left != null) traversal(node.left, res, level+1);
        if (node.right != null) traversal(node.right, res, level+1);
    }
}

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