Binary Search 的遞迴與迭代實現及STL中的搜尋相關內容
首先介紹一下binary search,其原理很直接,不斷地選取有序陣列的組中值,比較組中值與目標的大小,繼續搜尋目標所在的一半,直到找到目標,遞迴演算法可以很直觀的表現這個描述:
int binarySearchRecursive(int A[], int low, int high, int key) { if (low > high) return -1; int mid = (low + high) >> 1; if (key < A[mid]) return binarySearchRecursive(A, low, mid - 1, key);else if (key > A[mid]) return binarySearchRecursive(A, mid + 1, high, key); else return mid; }
但實際上,遞迴方法的時間效率和空間效率都不如迭代方法,迭代方法才是常用的binary search,程式碼如下:
int binarySearch(int A[], int low, int high, int key) { int mid; while (low <= high) { mid = (low + high) >> 1; if (key < A[mid]) high = mid - 1; else if (key > A[mid]) low = mid + 1; else return mid; } return -1; }
簡單計算一下Binary Search的效率:
演算法流程:
1.檢查上下邊界--2.獲取中值--3.比較--左半邊進入子問題/右半邊進入自問題/獲得結果
1,2所需時間為常數時間,設為C。3階段以一半的資料量重新執行函式,所以:
T(n)=T(n/2)+C
設n=2^k,則有T(2^k)=T(2^(k-1))+C=(T(2^(k-2))+C)+C=T(1)+k*C
即T(n)=log(n)*C+T(1),所以binary search是一個O(log(n))的演算法。
測試函式:
void searchTest() { int a[] = { 1,2,3,4,5,7,9,11,17,20,23,39 }; cout << binarySearch(a, 0, 11, 39) << endl; cout << binarySearch(a, 0, 11, 37) << endl; cout << binarySearch(a, 5, 10, 39) << endl; cout << endl; cout << binarySearchRecursive(a, 0, 11, 39) << endl; cout << binarySearchRecursive(a, 0, 11, 37) << endl; cout << binarySearchRecursive(a, 5, 10, 39) << endl; }
測試結果如下:
11
-1
-1
11
-1
-1
請按任意鍵繼續. . .
傳統C函式中有bsearch這一函式,因為在現代C++中使用C庫執行效率很低,加上介面並不好用,不再提及。而STL中,有以下幾個關於搜尋的函式。他們均作用於各個STL容器。
int count(起始迭代器,終止迭代器,key value)
return key value的數量
iterator find(起始迭代器,終止迭代器,key value)
成功:return 找到的第一個key value的迭代器
失敗:return 終止迭代器
bool binary_search(起始迭代器,終止迭代器,key value)
return 是否找到
iterator lower_bound(起始迭代器,終止迭代器,key value)
return 大於或等於key value的第一個迭代器,若所有值都小於key value,返回終止迭代器
iterator upper_bound(起始迭代器,終止迭代器,key value)
return 大於key value的第一個迭代器,若所有值都小於key value,返回終止迭代器
這些函式中,count和find是作用於任意排序物件的,其效率為O(n),而binary_search, lower_bound, upper_bound是作用於有序物件的,其效率是O(logN)。
下面程式碼給出這些STL函式的測試:
void searchTest() { vector<int> b{ 1,2,3,4,4,7,9,11,17,20,23,39 }; cout << "vector<int> b{ 1,2,3,4,4,7,9,11,17,20,23,39 };" << endl; cout << "count(b.begin(), b.end(), 4):" << count(b.begin(), b.end(), 4) << endl; cout << endl; cout << "find(b.begin(), b.end(), 39) - b.begin():" << find(b.begin(), b.end(), 39) - b.begin() << endl; cout << "find(b.begin(), b.end(), 4) - b.begin():" << find(b.begin(), b.end(), 4) - b.begin() << endl; cout << "find(b.begin(), b.end(), 37) - b.begin():" << find(b.begin(), b.end(), 37) - b.begin() << endl; cout << "find(b.begin() + 5, b.begin() + 10, 39) - b.begin():" << find(b.begin() + 5, b.begin() + 10, 39) - b.begin() << endl; cout << endl; cout << "binary_search(b.begin(), b.end(), 39):" << binary_search(b.begin(), b.end(), 39) << endl; cout << "binary_search(b.begin(), b.end(), 37):" << binary_search(b.begin(), b.end(), 37) << endl; cout << endl; cout << "lower_bound(b.begin(), b.end(), 39) - b.begin():" << lower_bound(b.begin(), b.end(), 39) - b.begin() << endl; cout << "lower_bound(b.begin(), b.end(), 4) - b.begin():" << lower_bound(b.begin(), b.end(), 4) - b.begin() << endl; cout << "lower_bound(b.begin(), b.end(), 37) - b.begin():" << lower_bound(b.begin(), b.end(), 37) - b.begin() << endl; cout << "lower_bound(b.begin() + 5, b.begin() + 10, 39) - b.begin():" << lower_bound(b.begin() + 5, b.begin() + 10, 39) - b.begin() << endl; cout << endl; cout << "upper_bound(b.begin(), b.end(), 39) - b.begin():" << upper_bound(b.begin(), b.end(), 39) - b.begin() << endl; cout << "upper_bound(b.begin(), b.end(), 4) - b.begin():" << upper_bound(b.begin(), b.end(), 4) - b.begin() << endl; cout << "upper_bound(b.begin(), b.end(), 37) - b.begin():" << upper_bound(b.begin(), b.end(), 37) - b.begin() << endl; cout << "upper_bound(b.begin() + 5, b.begin() + 10, 39) - b.begin():" << upper_bound(b.begin() + 5, b.begin() + 10, 39) - b.begin() << endl; }
測試結果:
vector<int> b{ 1,2,3,4,4,7,9,11,17,20,23,39 };
count(b.begin(), b.end(), 4):2
find(b.begin(), b.end(), 39) - b.begin():11
find(b.begin(), b.end(), 4) - b.begin():3
find(b.begin(), b.end(), 37) - b.begin():12
find(b.begin() + 5, b.begin() + 10, 39) - b.begin():10
binary_search(b.begin(), b.end(), 39):1
binary_search(b.begin(), b.end(), 37):0
lower_bound(b.begin(), b.end(), 39) - b.begin():11
lower_bound(b.begin(), b.end(), 4) - b.begin():3
lower_bound(b.begin(), b.end(), 37) - b.begin():11
lower_bound(b.begin() + 5, b.begin() + 10, 39) - b.begin():10
upper_bound(b.begin(), b.end(), 39) - b.begin():12
upper_bound(b.begin(), b.end(), 4) - b.begin():5
upper_bound(b.begin(), b.end(), 37) - b.begin():11
upper_bound(b.begin() + 5, b.begin() + 10, 39) - b.begin():10
請按任意鍵繼續. . .