Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked List

问题：

Given a binary tree, flatten it to a linked list in-place.

思路：

　　前序pre-order遍历+prenode

我的代码：

public class Solution {
    public void flatten(TreeNode root) {
        if(root == null)    return;
        if(pre != null)
        {
            pre.left = null;
            pre.right = root;
        }
        pre = root;
        TreeNode left = root.left;
        TreeNode right = root.right;
        flatten(left);
        flatten(right);
    }
    TreeNode pre = null;
}

学习之处：

• 转换成常见的前序遍历问题，然后再加上一个prenode指针就齐活了。