[LeetCode]Flatten Binary Tree to Linked List

题目描述:(链接)

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

解题思路：

/**
 * 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 flatten(TreeNode* root) {
        if (root == nullptr) return;
        
        stack<TreeNode *> cache;
        cache.push(root);
        while (!cache.empty()) {
            auto p = cache.top();
            cache.pop();
            
            if (p->right) {
                cache.push(p->right);
            }
            
            if (p->left) {
                cache.push(p->left);
            }
            
            p->left = nullptr;
            if (!cache.empty()) {
                p->right = cache.top();
            }
            
        }
        
    }
};