Convert Sorted List to Binary Search Tree 将链表转为二叉搜索树

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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:

    int cal(ListNode *root)
    {
        int len=0;
        while(root)
        {
            len++;
            root=root->next;
        }
        return len;
    }
    
    
    TreeNode *sortedListToBST(ListNode *head) {
        
        int len=cal(head);
        return solve(head,0,len-1);
    }
    
    TreeNode *solve(ListNode *head,int left,int right)
    {
        if(left > right) 
            return NULL;  
       int mid = (left+right)/2;  
       TreeNode *leftNode=solve(head, left, mid-1);
       
       for(int i=left;i<mid;i++)
             head=head->next;
       TreeNode *rightNode=solve(head->next, mid+1,right); 
       TreeNode *root=new TreeNode(head->val);  
       root->left=leftNode;  
       root->right =rightNode;  
       
       return root;  
    }
};