数据结构与算法：二分查找的C++实现

任务：分别用递归和循环两种方式实现二分查找算法（在某有序数组中查找特定的数字并返回其在数组中的下标）

代码如下：

#include <iostream>
#define NUM 20

using namespace std;

void Print(int* a, int len);	// 显示当前数组

void BinarySearch_Recursion(int* a, int left,int right,int value);	// 二分查找（递归实现）

void BinarySearch_Loop(int* a, int left, int right, int value);		// 二分查找（循环实现）


int main()
{
	int a[NUM];
	for (int i = 0; i < NUM; i++)
	{
		a[i] = 3 * i + 1;
	}
	cout << "数组a: ";
	Print(a, NUM);

	BinarySearch_Recursion(a, 0, NUM, 55);
	BinarySearch_Loop(a, 0, NUM, 31);

	while (1);
	return 0;
}


// 显示当前数组
void Print(int* a, int len)
{
	for (int i = 0; i < len; i++)
	{
		cout << a[i] << " ";
	}
	cout << endl;
}


// 二分查找（递归实现）
// a:被查找数组   left,right:二分查找的下标范围   value:查找的数值
void BinarySearch_Recursion(int* a, int left, int right, int value)
{
	
	// 查找合理性检查：left应该小于right
	if (left > right)
	{
		cout << "ERROR!"<< endl;
		return;
	}

	// 当left==right时，直接比较当前下标是否符合要求
	if (left == right)
	{
		if (a[left] == value)
		{
			cout << ">>> value: " << value << "	index: " << left << endl;
			return;
		}
		else
		{
			cout << ">>> No such value: " << value << endl;
			return;
		}
	}

	//递归：取下标范围中间处的值，若等于value，则输出结果，否则更改下标范围继续递归
	int mid = (left + right) / 2;
	if (a[mid] == value)
	{
		cout << ">>> value: " << value << "	index: " << mid << endl;
		return;
	}
	if (a[mid] > value)
	{
		BinarySearch_Recursion(a, left, mid, value);
	}
	else
	{
		BinarySearch_Recursion(a, mid+1, right, value);
	}
}


// 二分查找（循环实现）
// a:被查找数组   left,right:二分查找的下标范围   value:查找的数值
void BinarySearch_Loop(int* a, int left, int right, int value)
{
	// 查找合理性检查：left应该小于right
	if (left > right)
	{
		cout << "ERROR!" << endl;
		return;
	}

	// 当left==right时，直接比较当前下标是否符合要求
	if (left == right)
	{
		if (a[left] == value)
		{
			cout << ">>> value: " << value << "	index: " << left << endl;
			return;
		}
		else
		{
			cout << ">>> No such value: " << value << endl;
			return;
		}
	}

	//循环：取下标范围中间处的值，若等于value，则跳出循环输出结果，否则更改下标范围继续循环
	int mid = (left + right) / 2;
	while (a[mid] != value && left!=right)
	{
		mid = (left + right) / 2;
		if (a[mid] > value)
		{
			right = mid;
		}
		else
		{
			left = mid+1;
		}
		mid = (left + right) / 2;
	}

	if (a[mid] == value)
	{
		cout << ">>> value: " << value << "	index: " << mid << endl;
		return;
	}
	else
	{
		cout << ">>> No such value: " << value << endl;
		return;
	}
}