二分查找

// binarysearch.h
template <typename T>
int binarysearch(T val, T data[], int begin, int end, bool flag)
{
	if(flag){ // data 为升序数组
		if(val< data[begin] || val > data[end]){
			return -1;
		}
		int medidx = (begin+end)/2;
		
		if(val == data[medidx]){
			return medidx;
		}else if(val > data[medidx]){
			binarysearch(val, data, medidx + 1, end, flag);
		}else{
			binarysearch(val, data, begin, medidx - 1, flag);
		}

	}else{ // data为降序数组
		if(val > data[begin] || val < data[end]){
			return -1;
		}
		int medidx = (begin+end)/2;
		
		if(val == data[medidx]){
			return medidx;
		}else if(val > data[medidx]){
			binarysearch(val, data, begin, medidx - 1, flag);
		}else{
			binarysearch(val, data, medidx + 1, end, flag);
		}
	}
	cout << "Error input, check up and run again!\n";  
	return -1;
}

test.cpp

#include <iostream>
#include <iomanip>
#include <limits>
using std::cout;
using std::endl;
#include "binarysearch.h"

int main()
{
	//int data[] = {1, 2, 5, 7, 14, 15, 18, 21, 24, 27, 28, 33, 39, 41, 57, 66};
	//int sz = sizeof(data)/sizeof(data[0]);

	//int val = 77;
	//int idx = binarysearch(val ,data, 0, sz-1);

	//if(idx != -1){
	//	cout << "find " << val << " at index " << idx << endl;
	//}else{
	//	cout << val << " not found!\n";
	//}


	int data2[] = {100, 97, 92,89,87,86,85,81,74,72,70,63,61,55,48,47,41,30,23,17,15,7};
	int sz2 = sizeof(data2)/sizeof(data2[0]);
	int val2 = 100;

	int idx2 = binarysearch(val2, data2, 0, sz2 -1 , false);

	if(idx2 != -1){
		cout << "find " << val2 << " at index " << idx2 << endl;
	}else{
		cout << val2 << " not found!\n";
	}

	return 0;
}