103. Binary Tree Zigzag Level Order Traversal

Given a binary tree, return thezigzag level ordertraversal 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]
]

Analysis:

BFS level order traversal, count to record how many node in a entire level when last count reaches zero. int level to record which level it is on, mod 2 to determine if it is reverse order or not. Have a helper method to do reverse.

Code:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if(root == null)
        {
            //List<List<Integer>> returnList = new ArrayList<List<Integer>>();
            return new ArrayList<List<Integer>>();
        }

        Deque<TreeNode> queue = new LinkedList<TreeNode>();
        int level = 0, count = 1;
        queue.offer(root);
        List<Integer> levelList = new ArrayList<Integer>();
        List<List<Integer>> returnList = new ArrayList<List<Integer>>();
        while(!queue.isEmpty())
        {
            TreeNode cur = queue.poll();
            levelList.add(cur.val);
            count --;

            if(cur.left != null)
            {
                queue.offer(cur.left);
            }
            if(cur.right != null)
            {
                queue.offer(cur.right);
            }

            if(count == 0)
            {
                level++;
                count = queue.size();
                addNewLevel(levelList, returnList, level);
                levelList = new ArrayList<Integer>();
            }


        }
        return returnList;
    }

    private void addNewLevel(List<Integer> listToAdd, List<List<Integer>> returnList, int level)
    {
        if(level % 2 == 1)
        {
            returnList.add(listToAdd);
        }
        else
        {
            List<Integer> newList = new ArrayList<Integer>();
            for(int i = listToAdd.size()-1; i>=0; i--)
            {
                newList.add(((ArrayList<Integer>)listToAdd).get(i));
            }
            returnList.add(newList);
        }
    }
}