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

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

方法一：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/*
struct ListNode{
     int val;
     struct ListNode *next;
};*/

void firstView(struct TreeNode* root, struct ListNode **list){
    if(!root) return;
    (*list)->val = (int)root;
    (*list)->next = malloc(sizeof(struct ListNode));
    (*list)->next->val = 0;
    (*list)->next->next = NULL;
    (*list) = (*list)->next;
    firstView(root->left, list);
    firstView(root->right, list);
}

void flatten(struct TreeNode* root){
    struct ListNode *ptmp = NULL;
    struct ListNode list = {.val=0,.next=NULL};
    struct ListNode *p;

    list.next = malloc(sizeof(struct ListNode));
    list.next->val = 0;
    list.next->next = NULL;
    p = list.next;

    firstView(root, &p);
    p = list.next->next;
    while(p&&p->val){
        root->left = NULL;
        root->right = (struct TreeNode*)p->val;
        root = root->right;
        ptmp = p;
        p = p->next;
        free(ptmp);
    }
    free(list.next);
}