LeetCode:Flatten Binary Tree to Linked List

题目描述：

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

思路：在flattened tree中，节点的左子节点都为空，右子节点为二叉树先序遍历序列中该节点的前一个节点。所以可以先序遍历二叉树，在遍历的过程中维护一个next指针，一个pre指针。next指针指向当前遍历节点，pre指针指向前一个遍历的节点，令pre->right = next,pre->left = NULL。先序遍历结束后，即可得到要求的flattened tree。

代码：

void flatten(TreeNode *root)
{
if(root == NULL)
return;

stack<TreeNode *> treenode_stack;
TreeNode * pre;
TreeNode * next;
pre = root;
next = NULL;

if(root->right != NULL)
treenode_stack.push(root->right);
if(root->left != NULL)
treenode_stack.push(root->left);

while(!treenode_stack.empty())
{
TreeNode * node = treenode_stack.top();
treenode_stack.pop();
next = node;
pre->right = next;
pre->left = NULL;
pre = node;
if(node->right != NULL)
treenode_stack.push(node->right);
if(node->left != NULL)
treenode_stack.push(node->left);
}
}