[LeetCode]Convert Sorted List to Binary Search Tree

Convert Sorted List to Binary Search Tree

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

和上一题Convert Sorted Array to Binary Search Tree一样的道理，唯一的区别就是链表，不能直接取任意值。

注意取链表某个值的方法，就可以写出来。方法在纸上自己画下图就明白了。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * 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:
    int lengthOfList(ListNode* head)
    {
        int len = 0;
        while(head)
        {
            len++;
            head = head->next;
        }
        return len;
    }
    TreeNode* createTree(ListNode* head,int left,int right)
    {
        if(left>right) return NULL;
        ListNode* p = head;
        int mid = (left+right)/2;
        for(int i=left;i<mid;i++) p=p->next;
        TreeNode* leftTree = createTree(head,left,mid-1);
        TreeNode* rightTree = createTree(p->next,mid+1,right);
        TreeNode * root = new TreeNode(p->val);
        root->left = leftTree;
        root->right = rightTree;
        return root;
    }
    TreeNode* sortedListToBST(ListNode* head) {
        int n = lengthOfList(head);
        createTree(head,0,n-1);
    }
};