Question
You are given a sequence of n integers S and a sequence of different q integers T. Write a program which outputs C, the number of integers in T which are also in the set S.
Input
In the first line n is given. In the second line, n integers are given. In the third line q is given. Then, in the fourth line, q integers are given.
Output
Print C in a line.
Constraints
n ≤ 10000
q ≤ 500
0 ≤ an element in S ≤ 109
0 ≤ an element in T ≤ 109
Sample Input 1
5
1 2 3 4 5
3
3 4 1
Sample Output 1
3
Sample Input 2
3
3 1 2
1
5
Sample Output 2
0
Sample Input 3
5
1 1 2 2 3
2
1 2
Sample Output 3
2
Notes
Meaning
题目很简单,但是这里主要讲一个:线性搜索在引入标记之后,效率能提升常数倍。
Coding
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#include<iostream> #include<cstdio> #include<stdio.h> using namespace std; //带有标记的线性搜索 int search(int A[],int n, int key) { int i = 0; A[n] = key;//标记搜索先给关键字放在末尾 while (A[i] != key) i++; return i != n; } int A[100005]; int main() { int n, q, key,sum=0; scanf("%d", &n); for (int i = 0; i < n; i++) scanf("%d", &A[i]); scanf("%d", &q); for (int i = 0; i < q; i++) { scanf("%d", &key); if (search(A, n, key)) sum++; } printf("%d\n", sum); } |
Summary
向线性搜索种引入“标记”能够以将算法效率提升常数倍。
原始:
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LinearSearch() for i 从 0 到n-1 if A[i] 与key相等 return i returnNOT_FOUND |
改进后,给要查找的数放在数组末尾,用作标记。
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int search(int A[],int n, int key) { int i = 0; A[n] = key;//标记搜索先给关键字放在末尾 while (A[i] != key) i++; return i != n; } |