109. Convert Sorted List to Binary Search Tree

最新推荐文章于 2020-04-13 06:45:19 发布

shanshanhi

最新推荐文章于 2020-04-13 06:45:19 发布

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分类专栏： leetcode-list 文章标签： leetcode 109 list

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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.

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参考链接：
方法一、http://www.2cto.com/kf/201402/282179.html
方法二、http://www.cnblogs.com/ganganloveu/p/4061909.html
更简洁的博客：http://blog.youkuaiyun.com/worldwindjp/article/details/39722643

//时间复杂度为O（nlgn）的方法
ListNode* findMid(ListNode* head)
    {
        ListNode* slowTail = NULL;
        ListNode* slow = head;
        ListNode* fast = head;

        while(fast)
        {
            fast = fast->next;
            if(fast)
            {
                fast = fast->next;
                slowTail = slow;
                slow = slow->next;
            }
        }

        slowTail->next = NULL;//把链表分为两个部分
        return slow;
    }

    TreeNode* sortedListToBST(ListNode* head) {
        if(NULL == head)
        {
            return NULL;
        }
        else if(NULL == head->next)
        {
            return new TreeNode(head->val); //改成这种写法，而不是TreeNode* temp; temp->val = head->val; return temp;则就不会报runtime 错误
        }

        ListNode* mid = findMid(head);
        TreeNode* root = new TreeNode(mid->val);
        root->left = sortedListToBST(head);
        root->right = sortedListToBST(mid->next);
        return root;
    }