折半演算法，也就是二分尋找法

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一：用遞迴演算法

  /// <summary>        ///         /// </summary>        /// <param name="num">數組</param>        /// <param name="curr">被尋找的數</param>        /// <param name="count">需要尋找的索引位置</param>        /// <param name="isAfter">true 向前，false 向後</param>        /// <returns></returns>        public static int change(int[] num, int curr, int count, bool isAfter)        {            int index = count;            if (isAfter)                index = Convert.ToInt32(Math.Ceiling(count / 2d));            //判斷索引是否越界            if (index >= num.Length)            {                return -1;            }            if (num[index] < curr) //後半部分尋找            {                return change(num, curr, index + num.Length, true);            }            else if (num[index] > curr) //前半部分尋找            {                return change(num, curr, index - 1, false);            }            else if (curr == num[index])            {                return index;            }            return -1;        }

 

二：用while迴圈尋找

 /// <summary>        ///         /// </summary>        /// <param name="num">需要尋找的數組</param>        /// <param name="curr">尋找的數</param>        /// <returns></returns>        public static int change2(int[] num, int curr)        {            int count = num.Length;            int index = Convert.ToInt32(Math.Ceiling(count / 2d));            while (true)            {                //判斷索引是否越界                if (index >= num.Length)                {                    return -1;                }                if (num[index] < curr)                {                    count = index + num.Length;                    index = Convert.ToInt32(Math.Ceiling(count / 2d));                    continue;                }                else if (num[index] > curr)                {                    count = index - 1;                    index = count;                    continue;                }                else if (curr == num[index])                {                    return index;                }            }        }

 

 

 

 

 

測試：

 int[] num = { 1, 2, 8, 9, 95, 98, 100, 190, 900 };

            int results = change(num, 100, num.Length, false);

            int resultss = change2(num, 100);

 

折半演算法，也就是二分尋找法