Binary Tree Zigzag Level Order Traversal

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).

Example

Given binary tree {3,9,20,#,#,15,7},

3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]

Solution: BFS

public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
    // write your code here
    if(root == null)
        return new ArrayList<>();
    ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> item = new ArrayList<Integer>();
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    queue.add(root);
    boolean reverse = false;
    int cur = 1;
    int next = 0;
    while(!queue.isEmpty()) {
        TreeNode tmp = queue.poll();
        cur--;
        item.add(tmp.val);
        if(tmp.left != null) {
            queue.add(tmp.left);
            next++;
        }
        if(tmp.right != null) {
            queue.add(tmp.right);
            next++;
        }
        if(cur == 0) {
            cur = next;
            next = 0;
            if(!reverse) {
                res.add(item);
                reverse = true;
            }
            else {
                Collections.reverse(item);
                res.add(item);
                reverse = false;
            }
            item = new ArrayList<Integer>();
        }
    }
    return res;
}