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

 1 // To Compile and Run: g++ binary_search.cc -std=c++11 -Wall -O3 && ./a.out 8
 2
 3
 4 #include <stdlib.h>
 5 #include <assert.h>
 6
 7 #include <iostream>
 8 #include <vector>
 9
10
11 int BinarySearch(const std::vector<int> &inArr, int target) {
12     int left = 0;
13     int right = inArr.size() - 1;
14
15     while (true) {
16         if (left > right) {
17             std::cout << "Failed to find: " << target << "\n";
18             return -1;
19         }
20
21         int mid = left + (right - left) / 2;
22         if (target == inArr[mid]) {
23             return mid;
24         }
25
26         if (target > inArr[mid]) {
27             right = mid - 1;
28         }
29         // target < inArr[mid]
30         else {
31             left = mid + 1;
32         }
33     }
34
35     std::cerr << "Could never reach here.\n";
36     return -1;
37 }
38
39 int main(int argc, char const *argv[]) {
40     const std::vector<int> arr {
41         8, 7, 4, 3, -5, -9
42     };
43     assert(argc >= 2);
44     int target = atoi(argv[1]);
45
46     int indexOfTarget = BinarySearch(arr, target);
47     if (indexOfTarget < 0) {
48         std::cout << "Failed to find: " << target << "\n";
49         return -1;
50     }
51     std::cout << "Find: " << target << " in " << indexOfTarget << "\n";
52
53     return 0;
54 }

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：

 1 // To Compile and Run: g++ binary_search.cc -std=c++11 -Wall -O3 && ./a.out 8
 2
 3
 4 #include <stdlib.h>
 5 #include <assert.h>
 6
 7 #include <iostream>
 8 #include <vector>
 9
10
11 int BinarySearchRecursive(const std::vector<int> &inArr, int target, int left, int right) {
12     if (left > right) {
13         std::cout << "Failed to find: " << target << "\n";
14         return -1;
15     }
16
17     int mid = left + (right - left) / 2;
18     if (target == inArr[mid]) {
19         return mid;
20     }
21
22     if (target > inArr[mid]) {
23         right = mid - 1;
24     }
25     else {
26         left = mid + 1;
27     }
28
29     return BinarySearchRecursive(inArr, target, left, right);
30 }
31
32 int main(int argc, char const *argv[]) {
33     const std::vector<int> arr {
34         8, 7, 4, 3, -5, -9
35     };
36     assert(argc >= 2);
37     int target = atoi(argv[1]);
38
39     int indexOfTarget = BinarySearchRecursive(arr, target, 0, arr.size() - 1);
40     if (indexOfTarget < 0) {
41         std::cout << "Failed to find: " << target << "\n";
42         return -1;
43     }
44     std::cout << "Find: " << target << " in " << indexOfTarget << "\n";
45
46     return 0;
47 }