LeetCode Convert Sorted List to Binary Search Tree

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.

这题和上一题的用数组构建平衡二叉排序树比较类似，可以通过把List转换成Array之后在进行操作。具体用Array构建的方法见：

点击打开链接

但是这种办法的时间和空间复杂度都是O(n)；

这里参考网上的办法有一个空间复杂度O(n)，时间复杂度为O(1)的算法

TreeNode *sortedListToBST(ListNode *head){    int len = 0;        ListNode * node = head;        while (node != NULL)        {            node = node->next;            len++;        }        return buildTree(head, 0, len-1);    }        TreeNode *buildTree(ListNode *&node, int start, int end)    {        if (start > end) return NULL;        int mid = start + (end - start)/2;        TreeNode *left = buildTree(node, start, mid-1);        TreeNode *root = new TreeNode(node->val);        root->left = left;        node = node->next;        root->right = buildTree(node, mid+1, end);        return root;    }