动态规划

#include <iostream>  
#include <vector>

using namespace::std;
using std::cout;
using std::cin; 

vector<int> use;

int dfs(int now,int maxChoosableInteger,int desiredTotal)
{
    if(use[now] != 0)   
	{
        return use[now]==1;
	}
    int res=0;
    for(int i=maxChoosableInteger; i>0; i--)
	{
        int cur = 1<<(i-1);
        if((now & cur) != 0) 
		{
        	continue;
		} 
        if(i >= desiredTotal || !dfs(now|cur,maxChoosableInteger,desiredTotal-i))
		{
            res=1;
            break;
        }
    }
    use[now] = (res==true ? 1 : 2);
    return res;
    }
    
int sxyx(int maxChoosableInteger, int desiredTotal) 
{
    if(maxChoosableInteger >= desiredTotal)   
	{
    	return 1;
	}
    if(maxChoosableInteger*(maxChoosableInteger+1)/2 < desiredTotal) 
	{
    	return 0;
	}
    use = vector<int>(1<<maxChoosableInteger,0);
    return dfs(0,maxChoosableInteger,desiredTotal);
    }

int main() 
{
	int maxChoosableInteger,desiredTotal;
    cin >> maxChoosableInteger >> desiredTotal;
    if(sxyx(maxChoosableInteger,desiredTotal) == 1)
    {
    	std::cout << "true";
	}
	else
	{
		std::cout << "false";
	}
    return 0;  
}