搜索之二分

关于搜索，二分必提

/***如果数组中存在多个key,返回随机的那个key下标，如找不到返回-1**数组由小到大排列**版本1：递归二分搜索*/int binary_search1(int* array, int begin, int end, int key){if(begin > end)return -1;int mid = (begin+end)/2;if(array[mid] == key)return mid;else if(array[mid] > key)return binary_search1(array, begin, mid-1, key);else if(array[mid] < key)return binary_search1(array, mid+1, end, key);}/***如果数组中存在多个key,返回随机的那个key下标，如找不到返回-1**数组由小到大排列**版本2：非递归二分搜索*/int binary_search2(int* array, int begin, int end, int key){while(begin <= end){int mid = (begin+end)/2;if(array[mid] == key)return mid;else if(array[mid] > key)end = mid - 1;else if(array[mid] < key)begin = mid + 1;}return -1;}/***如果数组中存在多个key,返回下标最小的那个key，如找不到返回-1**数组由小到大排列**版本3：二分搜索(返回下标最小的那个key)*/int binary_search3(int* array, int begin, int end, int key){while(begin < end){int mid = (begin+end)/2;if(array[mid] == key)end = mid;else if(array[mid] > key)end = mid - 1;else if(array[mid] < key)begin = mid + 1;}if(array[begin] == key)return begin;elsereturn -1;}/***如果数组中存在多个key,返回下标最小的那个key，如找不到返回-1**数组由小到大排列**版本4：二分搜索(返回下标最小的那个key)**比版本3而言，优化了比较次数*/int binary_search4(int* array, int len, int key){int u = len;int l = -1;/*Start: 假象两个数array[l]与array[u] Assert: array[l] < key && array[u] >= key && u >= 0**/while(l + 1 != u){int mid = (l + u) / 2;if(array[mid] >= key)u = mid;elsel = mid;}/*Out: array[u] >= key && array[l] < key && l + 1 == u**/int index = u;if(index >= len || array[index] != key)return -1;return index;}int main(){int array[] = {3,3,3,3};int len = sizeof(array)/sizeof(int);//int index = binary_search3(array, 0, len-1, 3);int index = binary_search4(array, len, 3);printf("Find index is : %d/n", index );system("pause");return 0;}