二分查找的几种不同的实现

不多说，直接上代码，注释解释清楚了

# include <iostream>
# include <vector>
# include <cstring>
# include <string>
# include <sstream>
# include <istream>

using namespace std;

////////////////////////////////////////////////////////////////////
template <typename T>
vector <T> scanf_r() {
    cin.sync();
    string input_string;
    while(true) {
        getline(cin, input_string);
        if (input_string != "\n") break;
    }
    istringstream input_stream(input_string);
    vector <T> ret;
    T item;
    while(input_stream >> item) ret.push_back(item);
    return ret;
}

template <typename T>
T scanf(string prompt="", istream & input_stream = cin){
    T ret;
    while (cout << prompt, !(input_stream >> ret)) {
        input_stream.clear();  // reset the state of cin
        input_stream.sync(); // synchronize the buffer of cin
    }
    return ret;
}
//////////////////////////////////////////////////////////////////////

/**
 * 最小的i， 满足arr[i] = target. 不存在 返回 -1
 */
int bin_1(vector<int> & arr, int target) {
    int size = arr.size();
    int l = 0, r = size - 1, mid;
    while(l < r) {
        mid = l + ((r - l) >> 1); // 奇数： 中间； 偶数： 两个中间数的第一个
        if(arr[mid] < target)
            l = mid + 1;
        else // arr[i] >= target
            r = mid;
    }
    return (arr[l] == target) ? l : -1;
}


/**
 * 最大i， 满足arr[i] = target。 不存在， 返回-1
 */
int bin_2(vector<int> & arr, int target) {
    int size = arr.size();
    int l = 0, r = size - 1, mid;
    while(l < r) {
        mid = l + ((r - l + 1) >> 1); // 奇数： 中间；  偶数 ： 两个中间数的第二个
        if(arr[mid] > target)
            r = mid - 1;
        else // arr[i] <= target
            l = mid;
    }
    return (arr[l] == target) ? l : -1;
}


/**
 * 求最小的i，使得a[i] > target，若不存在，则返回-1
 */
int bin_3(vector<int> & arr, int target) {
    int size = arr.size();
    int l = 0, r = size - 1, mid;

    while(l < r) {
        mid = l + ((r - l) >> 1); 
        if(arr[mid] > target)
            r = mid;
        else
            l = mid + 1;
    }
    return (arr[l] > target) ? l : -1;
}


/**
 * 求最大的i，满足a[i] < target， 若不存在， 返回 -1
 */
int bin_4(vector<int> & arr, int target) {
    int size = arr.size();
    int l =0, r = size - 1, mid;
    while(l < r) {
        mid = l + ((r - l + 1) >> 1);
        if(arr[mid] < target)
            l = mid;
        else
            r = mid - 1;
    }
    return (arr[l] < target) ? l : -1;
}


/**
 * 求最小的i，满足a[i] >= target， 若不存在， 返回 -1
 */
int bin_5(vector<int> & arr, int target) {
    int size = arr.size();
    int l = 0, r = size - 1, mid;
    while(l < r) {
        mid = l + ((r - l) >> 1);
        if(arr[mid] >= target)
            r = mid;
        else
            l = mid + 1;
    }
    return (arr[l] >= target) ? l : -1;
}


/**
 * 求最大的i，满足a[i] <= target， 若不存在， 返回 -1
 */
int bin_6(vector<int> & arr, int target) {
    int size = arr.size();
    int l = 0, r = size - 1, mid;
    while(l < r) {
        mid = l + ((r - l + 1) >> 1);
        if(arr[mid] <= target)
            l = mid;
        else
            r = mid - 1;
    }
    return (arr[l] <= target) ? l : -1;
}


/////////////////////////////////////////////////////////////////////////////

void test( int (*bin) ( vector<int> &, int), string title) {
    cout << "------test " << title << " with target : 2 ----------" << endl;
    for(int i = 0; i < 3; i++) {
        vector<int> vec = scanf_r<int>();
        cout << bin(vec, 2) << endl;
    }
    cout << "------- end ---------------" << endl;
}



////////////////////////////////////////////////////////////////////////////

int main() {
    bool is_on = true;
    while(is_on) {
        cout << "choose a bin function to test: ";
        int key = scanf<int>();
        switch(key) {
            case 1:
                test(bin_1, string("bin_1(min i  for arr[i] = target)"));
                break;
            case 2:
                test(bin_2, string("bin_2(max i for arr[i] = target)"));
                break;
            case 3:
                test(bin_3, string("bin_3(min i for arr[i] > target)"));
                break;
            case 4:
                test(bin_4, string("bin_4(max i for arr[i] < target)"));
                break;
            case 5:
                test(bin_5, string("bin_5(min i for arr[i] >= target)"));
                break;
            case 6:
                test(bin_6, string("bin_6(max i for arr[i] <= target)"));
                break;
            case 0:
                is_on = false;
                break;
            default:;
        }

    }
    return 0;
}