LeetCode Binary Tree Zigzag Level Order Traversal

The serialization of a binary tree follows a level order traversal, where ‘#‘ signifies a path terminator where no node exists below.

Here‘s an example:

   1
  /  2   3
    /
   4
         5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

题意：层次遍历一颗树，奇数层的时候翻转。

思路：还是利用队列层次遍历，多一个判断是否翻转的标记就行了，还有就是java是引用传递的，所以每次都要重新声明一个对象，不然ans里面都是指向同一块内存。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<List<Integer>>();
    	if (root == null) return ans;
    	
    	Queue<TreeNode> queue = new LinkedList<TreeNode>();
    	queue.add(root);
    	boolean reverse = false;
    	while (!queue.isEmpty()) {
    		List<Integer> tmp = new ArrayList<Integer>();
    		int num = queue.size();
    		for (int i = 0; i < num; i++) {
    			TreeNode t = queue.poll();
    			tmp.add(t.val);
    			if (t.left != null) 
    				queue.add(t.left);
    			if (t.right != null) 
    				queue.add(t.right);
    		}
    		if (reverse) {
    			Collections.reverse(tmp);
    			reverse = false;
    		} else reverse = true;
    		ans.add(tmp);
    	}
    	
    	return ans; 
    }
}