34-二叉树中和为某一值的路径

输入一棵二叉树和一个整数，打印出二叉树中节点值的和为输入整数的所有路径。从树的根节点开始往下一直到叶节点所经过的节点形成一条路径。
示例:
给定如下二叉树，以及目标和 sum = 22，

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

返回:
[
[5,4,11,2],
[5,8,4,5]
]

class Solution {
    private List<List<Integer>> res = new ArrayList<>();
    private int expected;
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        if (root == null) return res;
        expected = sum;
        find(root, 0, new ArrayList<Integer>());
        return res;
    }

    private void find(TreeNode node, int current, List<Integer> list) {
        current += node.val;
        list.add(node.val);
        boolean isLeaf = node.left == null && node.right == null;
        if (isLeaf && current == expected) {
            res.add(new ArrayList(list));
        }
        if (node.left != null) find(node.left, current, list);
        if (node.right != null) find(node.right, current, list);
        list.remove(list.size() - 1);
    }
}