二分法查找. BinarySearch

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引言

前几天参加2014阿里巴巴春季校招（测试开发岗）笔试, 里面有一道改错题, 就是给了一个binary_search的代码. 结果这么基础的一题我还是没能答全对. 悲剧悲剧, 这个水平注定去不了阿里...现在开始亡羊补牢.

二分查找

算法: binarySearch

输入: 数组a, 起始索引begin, 结束索引end, 所查找值value

输出: 若找到则返回value在a中索引, 否则返回-1.

测试用例

后面两个版本的二分法都能通过以下用例:

0. a == NULL
1. 边界值, 即begin和end
2. 最后会剩两个数min, max. 查找key, key == min
3. 最后会剩两个数min, max. 查找max, key == max
4. 最后会剩两个数min, max. 查找key, key < min
5. 最后会剩两个数min, max. 查找key, min < key < max
6. 最后会剩两个数min, max. 查找key, key > max

如果有高人发现不能通过的用例还求不吝赐教. 跪谢.

算法binarySearch

自己先写一个:

int binarySearch(int* a, int begin, int end, int value){if(!a) return -1;int min = begin;int max = end;int middle;while(min <= max){middle = min + (max - min) / 2; //防止溢出，不写成mid = (max+min)/2if(a[middle] == value)return middle;else if(a[middle] < value){min = middle + 1;} else {max = middle - 1;}}return -1;}

编程之美版binarySearchBoP

int binarySearchBoP(int* arr, int begin, int end, int val){if(NULL == arr)return -1;int minIndex = begin;int maxIndex = end;int middleIndex;while(minIndex < maxIndex - 1){middleIndex = minIndex + (maxIndex - minIndex)/2; //防止溢出，不写成mid = (max+min)/2if(arr[middleIndex] <= val){minIndex = middleIndex;} else {maxIndex = middleIndex;}}if(arr[minIndex] == val){return minIndex;} else if(arr[maxIndex] == val){return maxIndex;} else {return -1;}}

完整程序

#include <iostream>#include <cstdlib>using namespace std;typedef int (*BSFuncPointer)(int*, int, int,  int);int binarySearch(int* a, int begin, int end, int value){if(!a) return -1;int min = begin;int max = end;int middle;while(min <= max){middle = min + (max - min) / 2;if(a[middle] == value)return middle;else if(a[middle] < value){min = middle + 1;} else {max = middle - 1;}}return -1;}int binarySearchBoP(int* arr, int begin, int end, int val){if(NULL == arr)return -1;int minIndex = begin;int maxIndex = end;int middleIndex;while(minIndex < maxIndex - 1){middleIndex = minIndex + (maxIndex - minIndex)/2;if(arr[middleIndex] <= val){minIndex = middleIndex;} else {maxIndex = middleIndex;}}if(arr[minIndex] == val){return minIndex;} else if(arr[maxIndex] == val){return maxIndex;} else {return -1;}}void test_binarySearch(BSFuncPointer binarySearch, int* a, int length, int value, int expectedResult){if (binarySearch(a, 0, length-1, value) == expectedResult){cout << "Test passed!" << endl;} else {cout << "Test failed with parameters[a, length, value, expectedResult]: " << a << ',' << length << ',' << value << ',' << expectedResult << endl; }}void test(BSFuncPointer func, const char* funcName, int* arr, int length){cout << "Test func: " << funcName << endl;// case0test_binarySearch(func, NULL, length, 0, -1); // case1test_binarySearch(func, arr, length, 1, 0); test_binarySearch(func, arr, length, 21, 10); // case2test_binarySearch(func, arr, length, 7, 3);// case3test_binarySearch(func, arr, length, 3, 1); // case4test_binarySearch(func, arr, length, 0, -1); // case5test_binarySearch(func, arr, length, 2, -1); // case6test_binarySearch(func, arr, length, 4, -1); }int main(){int arr[] = {1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21}; // 11个元素int length = sizeof(arr) / sizeof(int);test(binarySearch, "binarySearch", arr, length);test(binarySearchBoP, "binarySearchBoP", arr, length);system("pause");return 0;}

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