[LeetCode] 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.

解法：自底向上

　　   时间复杂度O(n), 空间复杂度O(logN)

class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        if (head == NULL) return NULL;
        
        int len = 0;
        ListNode *p = head;
        while (p) {
            ++len;
            p = p->next;
        }
        
        return sortedListToBST(head, 0, len - 1);
    }
    
    TreeNode* sortedListToBST(ListNode *&head, int start, int end) {
        if (start > end) return NULL;
        
        int mid = start + (end - start) / 2;
        TreeNode *left_child = sortedListToBST(head, start, mid - 1);
        TreeNode *root = new TreeNode(head->val);
        root->left = left_child;
        head = head->next;
        root->right = sortedListToBST(head, mid + 1, end);
        
        return root;
    }
};

 参考资料：

　　1.http://leetcode.com/2010/11/convert-sorted-list-to-balanced-binary.html