LeetCode Online Judge 题目C# 练习 - Search for a Range

Given a sorted array of integers, find the starting and ending position of a given target value.
Your algorithm's runtime complexity must be in the order of O(log n).
If the target is not found in the array, return [-1, -1].
For example,
Given [5, 7, 7, 8, 8, 10] and target value 8,
return [3, 4].

        public static List<int> SearchforaRange(int[] A, int target)
        {
            List<int> ret = new List<int>();

            ret.Add(BinarySearchlow(A, 0, A.Length - 1, target));
            ret.Add(BinarySearchup(A, 0, A.Length - 1, target));

            return ret;
        }

        public static int BinarySearchlow(int[] A, int start, int end, int target)
        {
            if (start > end)
                return -1;

            int mid = start + ((end - start) / 2);
            if(A[mid] == target && (mid == 0 || A[mid - 1] < target))
            {
                return mid;
            }
            else if (A[mid] < target)
            {
                return BinarySearchlow(A, mid + 1, end, target);
            }
            else
            {
                return BinarySearchlow(A, start, mid - 1, target);
            }
        }

        public static int BinarySearchup(int[] A, int start, int end, int target)
        {
            if (start > end)
                return -1;

            int mid = start + ((end - start) / 2);
            if (A[mid] == target && (mid == end || A[mid + 1] > target))
            {
                return mid;
            }
            else if (A[mid] > target)
            {
                return BinarySearchup(A, start, mid - 1, target);
            }
            else
            {
                return BinarySearchup(A, mid + 1, end, target);
            }
        }

代码分析：

　　时间复杂度要求O(log n)，已经提醒了要用二分查找法，这个其实就是一个Binary Search 的 Variant。