力扣 | 114. 二叉树展开为链表

class Solution {
    public void flatten(TreeNode root) {
           List<TreeNode> list = new ArrayList<TreeNode>();
           preorder(root, list);
        for (int i = 1; i < list.size(); i++) {
            TreeNode prev = list.get(i - 1);
            TreeNode curr = list.get(i);
            prev.left = null;
            prev.right = curr;
        }
    }

    public void preorder(TreeNode root, List<TreeNode> list) {
        if (root == null) return;
        list.add(root);
        preorder(root.left, list);
        preorder(root.right, list);
    }
}