109. Convert Sorted List to Binary Search Tree

题目地址
和上题108类似，关键在于分割当前链表，记得将左子链表尾部置NULL。

class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        if (!head) {
            return NULL;
        }
        int size = 0;
        ListNode *p = head;
        while (p) {
            size++;
            p = p->next;
        }
        ListNode *pre = NULL;
        p = head;
        for (int i = 0; i < size / 2; ++i) {
            pre = p;
            p = p->next;
        }
        TreeNode *root = new TreeNode(p->val);
        if (pre) {//有左孩子
            pre->next = NULL;
            root->left = sortedListToBST(head);
        }
        root->right = sortedListToBST(p->next);
        return root;
    }
};