递归---Flatten Binary Tree to Linked List

Flatten a binary tree to a fake “linked list” in pre-order traversal.

Here we use the right pointer in TreeNode as the next pointer in ListNode.

public class Solution {
private TreeNode lastNode = null;

public void flatten(TreeNode root) {
if (root == null) {
return;
}

if (lastNode != null) {
lastNode.left = null;
lastNode.right = root;
}

lastNode = root;
TreeNode right = root.right;
flatten(root.left);
flatten(right);
}
}