[LeetCode]Convert Sorted List to Binary Search Tree

题目描述:(链接)

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

解题思路:

/**
 * 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:
    TreeNode* sortedListToBST(ListNode* head) {
        return sortedListToBST(head, len(head));
    }
private:
    TreeNode *sortedListToBST(ListNode *head, int len) {
        if (len == 0) return nullptr;
        if (len == 1) return new TreeNode(head->val);
        
        int mid = len / 2;
        TreeNode *root = new TreeNode((afterNthNode(head, mid))->val);
        root->left = sortedListToBST(head, mid);
        root->right = sortedListToBST(afterNthNode(head, mid + 1), len - mid - 1);
        
        return root;
    }
    
    int len(ListNode *head) {
        int n = 0;
        while (head) {
            head = head->next;
            ++n;
        }
        
        return n;
    }
    
    ListNode *afterNthNode(ListNode *head, int n) {
        while (n--) {
            head = head->next;
        }
        
        return head;
    }
};