1010. Radix (25) PAT 甲级

#include<stdio.h>
#include<algorithm>
#include<string.h>
#include<string>
#include<math.h>

using namespace std;

typedef long long ll;

#define MAX_N 15

ll map[256];
ll INF=(1LL<<63)-1;

char N1[MAX_N],N2[MAX_N];
char temp[MAX_N];
int radix;
int tag;

ll toDecimal(char num[],int radix,ll top){
ll ans=0;
int l=strlen(num);
for(int i=0;i<l;i++){
ans=ans*radix+map[num[i]];
//printf("ans=%d\n",ans);
//printf("top=%lld\n",top);
if(ans<0||ans>top)  return -1;
}
return ans;
}

void initmap(){
for(char c='0';c<='9';c++){
map[c]=c-'0';
}
for(char c='a';c<='z';c++){
map[c]=c-'a'+10;
}
}

ll findLargestDigit(char num[]){
int ans=-1;
int l=strlen(num);
for(int i=0;i<l;i++){
if(map[num[i]]>ans){
ans=map[num[i]];
}
}
return ans+1;
}

int cmp(char N2[],ll radix,ll top){
//  printf("%s %d\n",N2,radix);
ll ans=toDecimal(N2,radix,top);
//printf("cmp ans=%d\n",ans);
//  printf("%d\n",top);
if(ans<0)
return -1;
if(top>ans)
return 1;
if(top==ans)
return 0;
return -1;
}

ll binarysearch(char N2[],ll low,ll high,ll top){
ll mid=0;
while(low<=high){
mid=(low+high)/2;
//printf("mid==%d\n",mid);
int flag=cmp(N2,mid,top);
if(flag==0)     return mid;
if(flag==-1)    high=mid-1;
if(flag==1)     low=mid+1;
}
return -1;
}

int main(){
initmap();
scanf("%s %s %d%d",N1,N2,&tag,&radix);
if(tag==2){
strcpy(temp,N1);
strcpy(N1,N2);
strcpy(N2,temp);
}
ll top=toDecimal(N1,radix,INF);
ll low=findLargestDigit(N2);
ll high=max(low,top)+1;
//printf("%d",high);
ll ans=binarysearch(N2,low,high,top);
if(ans==-1) printf("Impossible\n");
else    printf("%lld\n",ans);
}