103. Binary Tree Zigzag Level Order Traversal

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).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its zigzag level order traversal as:

Solution

BFS

/**
 * 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>> levelsList = new LinkedList();
        if (root == null) return levelsList;
        
        Queue<TreeNode> queue = new LinkedList();
        queue.add(root);
        boolean isReverse = false;
        while (!queue.isEmpty()) {
            int nodesCount = queue.size();
            List<Integer> levelList = new LinkedList();
            
            for (int i = 0; i < nodesCount; ++i) {
                TreeNode p = queue.poll();
                if (p.left != null) queue.add(p.left);  // filter null nodes
                if (p.right != null) queue.add(p.right);
                
                if (!isReverse) {
                    levelList.add(p.val);
                } else {
                    levelList.add(0, p.val);
                }
            }
            
            levelsList.add(levelList);
            isReverse = !isReverse;     // flip the order
        }
        
        return levelsList;
    }
}