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103. Binary Tree Zigzag Level Order Traversal - Medium

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]
]

 

M1: BFS

time: O(n), space: O(2^height)

/**
 * 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<>(); Queue<TreeNode> q = new LinkedList<>(); if(root == null) { return res; } boolean reverse = false; q.offer(root); while(!q.isEmpty()) { List<Integer> level = new
ArrayList<>(); int size = q.size(); for(int i = 0; i < size; i++) { TreeNode tmp = q.poll(); if(reverse) { level.add(0, tmp.val); } else { level.add(tmp.val); } if(tmp.left != null) { q.offer(tmp.left); } if(tmp.right != null) { q.offer(tmp.right); } } reverse = !reverse; res.add(level); } return res; } }

 

M2: recursion

time: O(n), space: O(height)

/**
 * 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;
        }
        levelOrder(root, 0, res);
        return res;
    }
    
    public void levelOrder(TreeNode root, int level, List<List<Integer>> res) {
        if(root == null) {
            return;
        }
        if(res.size() == level) {
            res.add(new ArrayList<>());
        }
        if(level % 2 == 0) {
            res.get(level).add(root.val);
        } else {
            res.get(level).add(0, root.val);
        }
        
        levelOrder(root.left, level + 1, res);
        levelOrder(root.right, level + 1, res);
    }
}