Binary Tree Zigzag Level Order Traversal

Problem

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). (link)

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

Better Solution

Perform a BFS traversal and reverse each array based on the reversal flag.

Time - O(n)

Space -O(n)

class Solution {
    private void reverse(List<Integer> arr){
        int n = arr.size();
        for(int i=0; i<n/2; i++){
            //swap i,n-i-1
            int j = n-i-1;
            int temp = arr.get(i);
            arr.set(i, arr.get(j));
            arr.set(j, temp);
        }
    }
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> zigZagList = new ArrayList<>();
        if(root==null) return zigZagList;
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        boolean reverseFlag = false;
        while(!queue.isEmpty()){
            List<Integer> levelOrder = new ArrayList<>();
            int size = queue.size();
            for(int i=0; i<size; i++){
                TreeNode node = queue.poll();
                if(node.left!=null) queue.add(node.left);
                if(node.right!=null) queue.add(node.right);
                levelOrder.add(node.val);
            }
            if(reverseFlag) reverse(levelOrder);
            reverseFlag = !reverseFlag;
            zigZagList.add(levelOrder);
        }

        return zigZagList;
    }
}

Optimal Solution

Perform BFS and add the level order elements to the list based on the reversal flag.

Time - O(n)

Space -O(n)

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> zigZagList = new ArrayList<>();
        if(root==null) return zigZagList;
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        boolean reverseFlag = false;
        while(!queue.isEmpty()){
            List<Integer> levelOrder = new ArrayList<>();
            int size = queue.size();
            for(int i=0; i<size; i++){
                TreeNode node = queue.poll();
                if(node.left!=null) queue.add(node.left);
                if(node.right!=null) queue.add(node.right);
                int index = reverseFlag ? 0 : levelOrder.size();
                levelOrder.add(index,node.val);
            }
            reverseFlag = !reverseFlag;
            zigZagList.add(levelOrder);
        }

        return zigZagList;
    }
}