LeetCode 103. Binary Tree Zigzag Level Order Traversal

Question

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

• Solution1 – BFS

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList<>();
        if(root != null) q.offer(root);
        while(!q.isEmpty()){
            int size = q.size();
            List<Integer> level = new ArrayList<>();
            for(int i = 0; i < size; i++){
                TreeNode cur = q.poll();
                if (res.size() % 2 == 0) level.add(cur.val);
                else level.add(0, cur.val);
                if(cur.left != null) q.offer(cur.left);
                if(cur.right != null) q.offer(cur.right);
            }
            res.add(level);
        }
        return res;
    }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)