Flatten Binary Tree to Linked List

https://leetcode.com/problems/flatten-binary-tree-to-linked-list/

二叉树先序遍历，之后做成链表

/**
 * 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==NULL)
            return;
        stack<TreeNode *> st1;
        st1.push(root);
        TreeNode * last=NULL;
        while(!st1.empty())
        {
            TreeNode * temp=st1.top();
            st1.pop();
            
            if(temp->right!=NULL)
                st1.push(temp->right);
            if(temp->left!=NULL)
                st1.push(temp->left);

            temp->left=NULL;
            temp->right=NULL;
            if(last!=NULL)
                last->right=temp;
            last=temp;
        }
    }
};