二分查找

注：区间是左闭右开的

普通二分查找

typedef int DataType;

int binary_search_any( DataType datas[], int begin, int end, DataType o)
{
    int midindex(0);
    while (begin < end)
    {
        midindex = (begin + end) >> 1;
        if (datas[midindex] < o)
            begin = midindex + 1;
        else if (datas[midindex] > o)
            end = midindex;
        else
            return midindex;
    }
    return -1;
}

查找最左边的元素——难点主要在于中间元素的处理上

int binary_search_first( DataType datas[], int begin, int end, DataType o)
{
    int midindex(0);
    while (begin < end)
    {
        midindex = (begin + end - 1) >> 1;
        if (datas[midindex] < o)
            begin = midindex +1;
        else if (datas[midindex] > o)
            end = midindex;
        else if ((datas[midindex] == o) && (begin + 1 == end))
            return midindex;
        else
            end = midindex + 1;
    }
    return -1;
}

查找最右边的元素

int binary_search_last( DataType datas[], int begin, int end, DataType o)
{
    int midindex(0);
    while (begin < end)
    {
        midindex = (begin + end) >> 1;
        if (datas[midindex] < o)
            begin = midindex + 1;
        else if (datas[midindex] > o)
            end = midindex;
        else if ((datas[midindex] == o) && (begin + 1 == end))
            return midindex;
        else
            begin = midindex;
    }
    return -1;
}

测试

#include <iostream>
#include <map>
#include <string>

using namespace std;


typedef int (*BS)( DataType [], int , int , DataType );

map<string, BS> sfs;

void test( DataType datas[], int begin, int end, DataType o)
{
    cout << "#####################################################" << endl;
    for(map<string, BS>::iterator  iter = sfs.begin(); iter != sfs.end(); ++ iter)
    {
        cout 
            << iter->first
            << "\t" 
            << o 
            << "\t" 
            << ( * iter->second)(datas, begin, end, o) 
            << endl;
    }
}

int main()
{
    DataType dts[] = {0,0,0,1,1,1,2,2,2};
    sfs.insert(pair<string, BS>("binary_search_any", &binary_search_any));
    sfs.insert(pair<string, BS>("binary_search_first", &binary_search_first));
    sfs.insert(pair<string, BS>("binary_search_last", &binary_search_last));
    test(dts, 0, 9, 0);
    test(dts, 0, 9, 1);
    test(dts, 0, 9, 2);
    test(dts, 0, 9, 3);
    test(dts, 0, 9, 10);
    test(dts, 0, 9, -1);
    return 0;
}