不兼容的类型：ArrayList <arraylist <integer>＆gt;无法转换为List <list <integer>＆gt;

Question

我正在做一个Leetcode问题＆＃34; Binary Tree Zigzag Level Order Traversal＆＃34;用java。 我收到了编译错误

  

＆＃34;不兼容的类型：ArrayList＆lt; ArrayList＆lt;整数＆GT;＆GT;不可能是   转换为List＆lt;列表＆lt;整数＆GT;＆GT;＆＃34;

任何人都可以告诉我错在哪里以及如何更改代码？ 非常感谢你！

以下是我的代码：

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

public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();

        if (root == null) return result;

        Stack<TreeNode> currLevel = new Stack<TreeNode>();
        Stack<TreeNode> nextLevel = new Stack<TreeNode>();
        Stack<TreeNode> tmp;

        currLevel.add(root);
        boolean normalOrder = true;

        while(!currLevel.isEmpty()) {
            ArrayList<Integer> currLevelResult = new ArrayList<Integer>();
            while(!currLevel.isEmpty()) {
                TreeNode node = currLevel.pop();
                currLevelResult.add(node.val);
                if (normalOrder) {
                    if(node.left != null) nextLevel.add(node.left);
                    if(node.right != null) nextLevel.add(node.right);
                }
                else {
                    if(node.right != null) nextLevel.add(node.right);
                    if(node.left != null) nextLevel.add(node.left);
                }
            }
            result.add(currLevelResult);
            tmp = currLevel;
            currLevel = nextLevel;
            nextLevel = tmp;
            normalOrder = !normalOrder;
        }
        return result;
    }
}