114. Flatten Binary Tree to Linked List -- 将二叉树转成链表（in-place单枝树）

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
Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

/**
 * 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) {
        stack<TreeNode*> s;
        TreeNode *p = root, *pre = NULL;
        while(p || !s.empty())
        {
            while(p)
            {
                if(pre)
                {
                    pre->right = p;
                    pre = NULL;
                }
                if(!p->left && !p->right)
                    pre = p;
                s.push(p);
                p = p->left;
            }
            if(!s.empty())
            {
                p = s.top();
                s.pop();
                p = p->right;
            }
        }
        for(p = root; p; p = p->right)
        {
            if(p->left)
            {
                p->right = p->left;
                p->left = NULL;
            }
        }
    }
};