• Exhaustive Search


    Write a program which reads a sequence A of n elements and an integer M, and outputs "yes" if you can make M by adding elements in A, otherwise "no". You can use an element only once.

    You are given the sequence A and q questions where each question contains Mi.

    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 (Mi) are given.

    Output

    For each question Mi, print yes or no.

    Constraints

    • n ≤ 20
    • q ≤ 200
    • 1 ≤ elements in A ≤ 2000
    • 1 ≤ Mi ≤ 2000

    Sample Input 1

    5
    1 5 7 10 21
    8
    2 4 17 8 22 21 100 35
    

    Sample Output 1

    no
    no
    yes
    yes
    yes
    yes
    no
    no
    

    Notes

    You can solve this problem by a Burte Force approach. Suppose solve(p, t) is a function which checkes whether you can make t by selecting elements after p-th element (inclusive). Then you can recursively call the following functions:

    solve(0, M) 
    solve(1, M-{sum created from elements before 1st element}) 
    solve(2, M-{sum created from elements before 2nd element}) 
    ...

    The recursive function has two choices: you selected p-th element and not. So, you can check solve(p+1, t-A[p]) and solve(p+1, t) in solve(p, t) to check the all combinations.

    For example, the following figure shows that 8 can be made by A[0] + A[2].

    #include <iostream>
    using namespace std;
    
    int n, q;
    int A[2010], M[2010];
    
    // 用第i个元素后面的元素能得出m时返回true 
    // 从输入值M中减去所选元素的递归函数 
    bool solve(int i, int m)
    {
    	if(m == 0)	return true;
    	if(i >= n)	return false;
    	return solve(i + 1, m) || solve(i + 1, m - A[i]);	// 不使用第i个元素 + 使用第i个元素 
    }
    
    int main()
    {
    	cin >> n;
    	for(int i = 0; i < n; ++ i)
    	{
    		cin >> A[i];
    	}
    	cin >> q;
    	for(int i = 0; i < q; ++ i)
    	{
    		cin >> M[i];
    		if(solve(0, M[i]))
    		{
    			cout << "yes" << endl;
    		}
    		else
    		{
    			cout << "no" << endl;
    		}
    	}
    	
    	return 0;
    }
    
    /*
    5
    1 5 7 10 21
    8
    2 4 17 8 22 21 100 35
    */
    

      

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  • 原文地址:https://www.cnblogs.com/mjn1/p/10745910.html
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