折半查找

C#

// --------------------------------------------------------------------------------------------------------------------
// <copyright company="Chimomo's Company" file="Program.cs">
// Respect the work.
// </copyright>
// <summary>
// The binary search (not recursive).
// [折半查找的前提]：
// 1、待查找序列必须采用顺序存储结构。
// 2、待查找序列必须是按关键字大小有序排列。
// </summary>
// --------------------------------------------------------------------------------------------------------------------

namespace CSharpLearning
{
    using System;

    /// <summary>
    /// The program.
    /// </summary>
    internal class Program
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        public static void Main()
        {
            int[] a = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            Console.WriteLine(BinarySearch(a, 6, 9));
        }

        /// <summary>
        /// 在长度为n的有序数组a中查找值为key的元素（非递归查找）。
        /// </summary>
        /// <param name="a">
        /// 待查找数组。
        /// </param>
        /// <param name="key">
        /// 目标元素。
        /// </param>
        /// <param name="n">
        /// 数组长度。
        /// </param>
        /// <returns>
        /// 若查找到目标元素则返回该目标元素在数组中的下标；否则返回-1。
        /// </returns>
        private static int BinarySearch(int[] a, int key, int n)
        {
            int low = 0;
            int high = n - 1;
            while (low <= high)
            {
                int mid = (low + high) / 2;
                if (a[mid] == key)
                {
                    return mid;
                }

                if (a[mid] < key)
                {
                    low = mid + 1;
                }
                else
                {
                    high = mid - 1;
                }
            }

            return -1;
        }
    }
}

// Output:
/*
5
*/