_103.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;


/**
 * 103. 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).

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]
]
*/
public class _103 {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        Queue<TreeNode> q = new LinkedList();
        List<List<Integer>> levels = new ArrayList();
        if (root == null) {
            return levels;
        }
        q.offer(root);
        boolean forward = true;
        while (!q.isEmpty()) {
            int size = q.size();
            List<Integer> level = new ArrayList();
            for (int i = 0; i < size; i++) {
                TreeNode curr = q.poll();
                level.add(curr.val);
                if (curr.left != null) {
                    q.offer(curr.left);
                }
                if (curr.right != null) {
                    q.offer(curr.right);
                }
            }
            if (forward) {
                forward = false;
                levels.add(level);
            } else {
                Collections.reverse(level);
                levels.add(level);
                forward = true;
            }
        }
        return levels;
    }
}