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]
]

Solution

• 关于Tree 和 Level，用BFS， 思路与102类似，用2个queue 或 stack
• 但是题目要求的是 zigzag level order， 先入后出，所以这里需要用stack.
• 而且，如果假设TREE只有一个根节点时，其层数为1。
  ---- 偶数层，在向stack2加入新节点时，先add right 再add left.
  ---- 奇数层，在向stack2加入新节点时，先add left 再add right.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<> ();
        if (root == null) {
            return result;
        }
        
        Stack<TreeNode> tracker1 = new Stack<> ();
        Stack<TreeNode> tracker2 = new Stack<> ();
        int layer = 1;
        List<Integer> subArray = new ArrayList<> ();
        
        tracker1.add (root);
        
        while (!tracker1.isEmpty ()) {
            TreeNode current = tracker1.pop ();
            subArray.add (current.val);
            
            if (layer % 2 != 0) {
                if (current.left != null) {
                    tracker2.push (current.left);
                }
                if (current.right != null) {
                    tracker2.push (current.right);
                }
            } else if (layer % 2 == 0) {
                if (current.right != null) {
                    tracker2.push (current.right);
                }
                
                if (current.left != null) {
                    tracker2.push (current.left);
                }
            }
            
            if (tracker1.isEmpty ()) {
                result.add (subArray);
                subArray = new ArrayList<Integer> ();
                
                tracker1 = tracker2;
                tracker2 = new Stack<TreeNode> ();
                layer++;
            }

        }
        
        return result;
    }
}