算法 - 折半查找（C#）
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// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// The binary search (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, 0, 9)); } /// <summary> /// 在下界为low，上界为high的有序数组a中折半查找数据元素x（递归查找）。 /// </summary> /// <param name="a"> /// 待查找数组。 /// </param> /// <param name="x"> /// 目标元素。 /// </param> /// <param name="low"> /// 数组元素下标的下界。 /// </param> /// <param name="high"> /// 数组元素下标的上界。 /// </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 x, int low, int high) { if (low > high) { return -1; } int mid = (low + high) / 2; if (x == a[mid]) { return mid; } return x < a[mid] ? BinarySearch(a, x, low, mid - 1) : BinarySearch(a, x, mid + 1, high); } }}// Output:/*5*/时间复杂度：O(log2n)
// --------------------------------------------------------------------------------------------------------------------// <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*/时间复杂度：O(log2n)
