[Leetcode]Flatten Binary Tree to Linked List

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

click to show hints.

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
void preorder(TreeNode* root,TreeNode* &tail){
if(!root)return;
TreeNode* l = root->left,*r = root->right;
if(tail)tail->right = root;
tail = root;
tail->left = NULL;
preorder(l,tail);
preorder(r,tail);
}
void flatten(TreeNode* root) {
//preorder
TreeNode* tail = NULL;
preorder(root,tail);
}
};

class Solution {
public:
void flatten(TreeNode* root) {
stack<TreeNode*>stk;
if(root)stk.push(root);
TreeNode* tail = NULL;
while(!stk.empty()){
TreeNode* t = stk.top();stk.pop();
if(t->right)stk.push(t->right);
if(t->left)stk.push(t->left);
if(tail)tail->right = t;
tail = t;
tail->left = NULL;
}
}
};