109. Convert Sorted List to Binary Search Tree --M

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

 A:
 就是用slow runner 和fast runner 找到中间node。然后，循环调用左边，右边即可。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedListToBST(ListNode* head) {
        if(!head)
            return nullptr;
        ListNode * header = new ListNode(0);
        header->next = head;            
        ListNode * preSlow = header;
        ListNode * fast = head;
        while(fast && fast->next){
            fast = fast->next->next;
            preSlow = preSlow->next;
        }
        TreeNode *pRoot = new TreeNode(preSlow->next->val); // slow->next is the middle point
        pRoot->right = sortedListToBST(preSlow->next->next);
        preSlow->next = nullptr;
        pRoot->left= sortedListToBST(header->next);
        delete header;
        return pRoot;
    }
};

这个的核心就是,起步的时候， preSlow 就比fast 提前一个位置。

Mistakes:
------------------------第一遍水过， 第二遍 结果出现错误了。哎，  --- 原因是， preSlow节点，刚开始设为head,但后来设为preSlow.next = null的时候， 如果原本的list只有2个node，就会丢掉后面的？？？？？？？？为什么呢？， 哦， 因为， preSlow那个时候，和slow指的是同一个对象！！！！！
--------最新方法，就是直接用了header， ---------好东西啊-----------  ^_^ 

Learned: