算法：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

class Solution {
public:
    void flatten(TreeNode* root) {
        
        if(root==NULL)
            return;
        
        stack node_stack;
        node_stack.push(root);
        TreeNode* new_root=NULL;
        TreeNode* cur=NULL;
        TreeNode* next=NULL;
        while(!node_stack.empty()){
            cur=node_stack.top();
            node_stack.pop();
            if(cur->right!=NULL)
                node_stack.push(cur->right);
            if(cur->left!=NULL)
                node_stack.push(cur->left);
            if(new_root==NULL){
                new_root=cur;
                next=cur;
                cur->left=NULL;
            }else {
                next->right=cur;
                next->left=NULL;
                next=cur;
            }
        }
        root=new_root;
    }
};