114. Flatten Binary Tree to Linked List

二叉树 -----前序遍历----> 链表

public void flatten(TreeNode root) {
    if (root == null) return;
    TreeNode left = root.left;
    TreeNode right = root.right;

    root.left = null;

    flatten(left);
    flatten(right);

    TreeNode cur = root;
    root.right = left;
    while (cur.right != null) {
        cur = cur.right;
    }
    cur.right = right;
}