LeetCode 114. Flatten Binary Tree to Linked List

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

For example,
Given

     1    / \   2   5  / \   \ 3   4   6

The flattened tree should look like:
1
\
2
\
3
\
4
\
5
\
6

开始，我遇到了一个大问题，总是进入死循环。为题在于，遍历左子树的时候，root.left 将 root.right中的信息给覆盖了，因此程序进入不断压栈的状态。因此，应该在进行遍历左子树的时候，将本节点的右子树的信息进行保存。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    TreeNode lastNode = null;    public void flatten(TreeNode root) {        if(root == null) {            return;        }        if (lastNode != null) {            lastNode.right = root;            lastNode.left = null;        }        lastNode = root;        TreeNode right = root.right;        flatten(root.left);        flatten(right);    }}

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