学步园

这题拿个20分左右还是比较简单地，拿满分花了很久，代码很丑。

关键要考虑二分和溢出问题，还是贴一下代码。

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
long long rest; 
//判断sum和tmp小于0考虑了溢出情况 
long long RETDEC(char str[],long long radix){
  long long i,j=0;
  long long sum=0;
  for(i=strlen(str)-1;i>=0;i--){
    if('0'<=str[i]&&str[i]<='9'){
      long long tmp=1,k=0;
      while(k<j){
        tmp=tmp*radix;
        if(tmp<0) return -1;
        k++;
      }
      sum=sum+(str[i]-'0')*tmp;
      if(sum<0) return -1;
    }
    if('a'<=str[i]&&str[i]<='z'){
      long long tmp=1,k=0;
      while(k<j){
        tmp=tmp*radix;
        if(tmp<0) return -1;
        k++;
      }
      sum=sum+(str[i]-'a'+10)*tmp;
      if(sum<0) return -1;
    }
    j++;
  }
  return sum;
}
int RETBIG(char str[]){
  int maxx=0,i;
  for(i=strlen(str)-1;i>=0;i--){
    if('0'<=str[i]&&str[i]<='9'){
      if((str[i]-'0')>maxx) maxx=str[i]-'0';
    }
    if('a'<=str[i]&&str[i]<='z'){
      if((str[i]-'a'+10)>maxx) maxx=str[i]-'a'+10;
    }
  }
  return maxx+1;
}
void binary(long long ans,char str[],long long start,long long end){
	if(start>end) return;
	long long middle=(start+end)/2;
	if(RETDEC(str,middle)<0||RETDEC(str,middle)>ans) binary(ans,str,start,middle-1);
	else if(RETDEC(str,middle)<ans) binary(ans,str,middle+1,end);
	else{
	     rest=middle;
	}
}
int main(){
  int tag,radix;
  long long i,ans;
  char N1[12],N2[12];
  scanf("%s %s %d %d",N1,N2,&tag,&radix);
  if(tag==1){
    ans=RETDEC(N1,radix);
    rest=-1;
    int lower=RETBIG(N2);
    binary(ans,N2,lower,ans+1);
  }
  else{
    ans=RETDEC(N2,radix);
    rest=-1;
    int lower=RETBIG(N1);
    binary(ans,N1,lower,ans+1);
  }
  if(rest==-1) printf("Impossible\n");
  else printf("%lld\n",rest);
  system("pause");
  return 0;
}