LeetCode | 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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

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

 //“之”字型层序遍历，思想参考层序遍历的1与2就能想明白
 //当此层不需要逆序时，使用thisLevelList.add(curNode.val)
 //当此层需要逆序时，使用thisLevelList.addFirst(curNode.val)来达到逆序的效果
 //使用标志位zigzag来判断本层需要逆序否，初始root对应false(不需要逆序)，每遍历完一层时改变zigzag的值
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();  
        if(root==null) return result;                        
        
        Queue<TreeNode> nodeQueue = new LinkedList<TreeNode>();
        nodeQueue.add(root);
        TreeNode nextLevelStarter;
        boolean zigzag = false;   //第一层root不需要逆序
        
        while(!nodeQueue.isEmpty()){
            LinkedList<Integer> thisLevelList = new LinkedList<Integer>();
            nextLevelStarter = null;
            
            while(nodeQueue.isEmpty()==false && nodeQueue.peek()!=nextLevelStarter){
                TreeNode curNode = nodeQueue.remove();
                
                if(!zigzag){                              //与普通层序遍历的主要区别
                    thisLevelList.add(curNode.val);       //不需要逆序
                }else{
                    thisLevelList.addFirst(curNode.val);  //需要逆序
                }
                
                if(curNode.left!=null) nodeQueue.add(curNode.left);
                if(curNode.right!=null) nodeQueue.add(curNode.right);
                
                if(nextLevelStarter==null){
                    if(curNode.left!=null){
                        nextLevelStarter = curNode.left;
                    }else if(curNode.right!=null){
                        nextLevelStarter = curNode.right;
                    }
                }
            } 
            zigzag = !zigzag;          //当本层遍历完之后，修改标志位zigzag以下层使用
            result.add(thisLevelList);     
    }
    
    return result;
  }
}