PAT (Advanced Level) 1103. Integer Factorization (30)

#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<map>
#include<queue>
#include<stack>
#include<algorithm>
using namespace std;

int n,k,p,top;
int path[1000];
int flag=0;
int ans_path[1000],ans_num=0,ans_len;

void dfs(int x,int pre,int sum,int num)
{
    if(sum>n) return;
    if(x>k) return;
    if(sum==n)
    {
        if(x!=k) return;

        flag=1;
        if(num>ans_num)
        {
            ans_num=num;
            ans_len=x;
            for(int i=0;i<x;i++) ans_path[i]=path[i];
        }

        return;
    }
    for(int i=pre;i>=1;i--)
    {
        path[x]=i;
        dfs(x+1,i,sum+int(pow(i,p)+0.5),num+i);
    }
}
int main()
{
    scanf("%d%d%d",&n,&k,&p);
    top=(int) (pow(1.0*n,1.0/p)+0.5);
    top=min(top,n/k); //加这个剪枝速度还可以提高一些

    dfs(0,top,0,0);
    if(flag==0) printf("Impossible\n");
    else
    {
        printf("%d = ",n);

        for(int i=0;i<ans_len;i++)
        {
            printf("%d^%d",ans_path[i],p);
            if(i!=ans_len-1) printf(" + ");
        }
    }
    return 0;
}