Binary Tree Zigzag Level Order Traversal.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;

public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> r = new ArrayList<List<Integer>>();
        ArrayDeque<TreeNode> q = new ArrayDeque<TreeNode>();
        if(root==null) return r;
        q.offer(root);
        boolean reversedOrder = false;
        while(!q.isEmpty()) {
            int len = q.size();
            List<Integer> level = new LinkedList<Integer>();
            for(int i=0;i<len;i++){
                TreeNode n = q.poll();
                if(reversedOrder)
                    level.add(0, n.val);
                else
                    level.add(n.val);
                if(n.left!=null) q.offer(n.left);
                if(n.right!=null) q.offer(n.right);
            }
            r.add(level);
            reversedOrder = !reversedOrder;
        }
        return r;
    }
}