Linear Search

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

#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：

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。

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;
}

​

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