LeetCode——convert-sorted-list-to-binary-search-tree && convert-sorted-array-to-binary-search-tree

Q：给定一个单链表，其中的元素按升序排序，请将它转化成平衡二叉搜索树（BST）
A：

    public static TreeNode sortedListToBST(ListNode head) {
        if (head == null)
            return null;
        return toBST(head, null);
    }

    private static TreeNode toBST(ListNode head, ListNode tail) {
        if (head == tail)
            return null;
        // 申请两个指针，fast移动速度是low的两倍
        ListNode fast = head;
        ListNode slow = head;
        while (fast != tail && fast.next != tail) {
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode root = new TreeNode(slow.val);
        root.left = toBST(head, slow);
        root.right = toBST(slow.next, tail);

        return root;
    }

Q：给出一个升序排序的数组，将其转化为平衡二叉搜索树（BST）.
A：

    public TreeNode sortedArrayToBST(int[] nums) {
        if(nums==null||nums.length==0)
            return null;
        return sortedArrayToBST(nums,0,nums.length-1);
    }
 
    private TreeNode sortedArrayToBST(int[] nums, int left, int right) {
        if(right<left)
            return null;
        if(left==right)
            return new TreeNode(nums[left]);
        int mid=left+(right-left+1)/2;
        TreeNode root=new TreeNode(nums[mid]);
         
        root.left=sortedArrayToBST(nums,left,mid-1);
        root.right=sortedArrayToBST(nums,mid+1,right);
         
        return root;
    }