BinarySearch，逆序排列的数组的二分查找（折半查找），C++非递归+递归实现

// To Compile and Run: g++ binary_search.cc -std=c++11 -Wall -O3 && ./a.out 8


#include <stdlib.h>
#include <assert.h>

#include <iostream>
#include <vector>


int BinarySearch(const std::vector<int> &inArr, int target) {
    int left = 0;
    int right = inArr.size() - 1;

    while (true) {
        if (left > right) {
            std::cout << "Failed to find: " << target << "\n";
            return -1;
        }

        int mid = left + (right - left) / 2;
        if (target == inArr[mid]) {
            return mid;
        }

        if (target > inArr[mid]) {
            right = mid - 1;
        }
        // target < inArr[mid]
        else {
            left = mid + 1;
        }
    }

    std::cerr << "Could never reach here.\n";
    return -1;
}

int main(int argc, char const *argv[]) {
    const std::vector<int> arr {
        8, 7, 4, 3, -5, -9
    };
    assert(argc >= 2);
    int target = atoi(argv[1]);

    int indexOfTarget = BinarySearch(arr, target);
    if (indexOfTarget < 0) {
        std::cout << "Failed to find: " << target << "\n";
        return -1;
    }
    std::cout << "Find: " << target << " in " << indexOfTarget << "\n";

    return 0;
}

    对于顺序排列的数组，只修改26行的符号就行了。

    使用递归实现，只需要在BinarySearch函数中去掉while循环，判断结束后，return自身就行了。

    代码如下：

// To Compile and Run: g++ binary_search.cc -std=c++11 -Wall -O3 && ./a.out 8


#include <stdlib.h>
#include <assert.h>

#include <iostream>
#include <vector>


int BinarySearchRecursive(const std::vector<int> &inArr, int target, int left, int right) {
    if (left > right) {
        std::cout << "Failed to find: " << target << "\n";
        return -1;
    }

    int mid = left + (right - left) / 2;
    if (target == inArr[mid]) {
        return mid;
    }

    if (target > inArr[mid]) {
        right = mid - 1;
    }
    else {
        left = mid + 1;
    }

    return BinarySearchRecursive(inArr, target, left, right);
}

int main(int argc, char const *argv[]) {
    const std::vector<int> arr {
        8, 7, 4, 3, -5, -9
    };
    assert(argc >= 2);
    int target = atoi(argv[1]);

    int indexOfTarget = BinarySearchRecursive(arr, target, 0, arr.size() - 1);
    if (indexOfTarget < 0) {
        std::cout << "Failed to find: " << target << "\n";
        return -1;
    }
    std::cout << "Find: " << target << " in " << indexOfTarget << "\n";

    return 0;
}