【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

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
void flatten(TreeNode *root) {
if(root == nullptr) {
return;
}
stack<TreeNode *> s;
s.push(root);
while (!s.empty()) {
TreeNode *p = s.top();
s.pop();
if (p->right != nullptr) {
s.push(p->right);
}
if (p->left != nullptr) {
s.push(p->left);
}
if (!s.empty()) {
p->right = s.top();
}
p->left = nullptr;
}
}
};

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