[LeetCode]Flatten Binary Tree to Linked List

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void flatten(TreeNode* root) {
13         if (root == nullptr) return;
14         
15         stack<TreeNode *> cache;
16         cache.push(root);
17         while (!cache.empty()) {
18             auto p = cache.top();
19             cache.pop();
20             
21             if (p->right) {
22                 cache.push(p->right);
23             }
24             
25             if (p->left) {
26                 cache.push(p->left);
27             }
28             
29             p->left = nullptr;
30             if (!cache.empty()) {
31                 p->right = cache.top();
32             }
33             
34         }
35         
36     }
37 };