1010. Radix (25)

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

#define max 11

char a[4][max];

long long num2Dec(char * p, long long radix)
{
long long  i;
long long  len=strlen(p);

long long digit = 0;
long long m = 1;
long long sum = 0;
for(i=len-1;i>=0;i--)
{
if(p[i]>='a'&&p[i]<='z')
digit= p[i] - 'a' + 10;
else if(p[i]>='0'&& p[i]<='9')
digit=p[i] - '0';
sum+=digit*m;
m*=radix;
}
return sum;
}

int findLeastRadix(char *p)
{
long long  len=strlen(p);
long long  low=0;
long long num;
long long i;
for(i=len-1;i>=0;i--)
{
if(p[i]>='a'&&p[i]<='z')
num= p[i] - 'a' + 10;
else if(p[i]>='0'&& p[i]<='9')
num=p[i] - '0';
if(num+1>low)
low=num+1;
}
return low;

}

int compare(char* p, long long radix, long long target)
{
long long  i;
long long len=strlen(p);

long long m = 1;
long long num = 1;
long long sum = 0;
for(i=len-1;i>=0;i--)
{
if(p[i]>='a'&&p[i]<='z')
num= p[i] - 'a' + 10;
else if(p[i]>='0'&& p[i]<='9')
num=p[i] - '0';
sum+=num*m;
m*=radix;
if(sum>target)  //avoid  overflow
return 1;
}

if(sum>target)
return 1;
else if(sum<target)
return -1;
else
return 0;

}

long long binarySearch(char* p, long long low, long long high, long long target){

long long mid = low;
int tag;

while(low<=high) {

tag = compare(p, mid, target);

switch(tag) {
case -1:
low = mid + 1;
break;
case 0:
return mid;
case 1:
high = mid - 1;
}

mid = (low + high) >> 1;
}

return -1;
}

int main(int argc, char* argv[])
{
int i;
int tag;
int base;
long long n1;
long long n2;
long long low;
long long high;
long long radix;

for(i=0; i<4; i++) {
scanf("%s", a[i]);
}

tag = atoi(a[2]);
base = atoi(a[3]);

switch(tag) {
case 1 :
n1 = num2Dec(a[0], base);
low=findLeastRadix(a[1]);
high = (n1+1 > low+1) ? n1+1 : low+1;
radix = binarySearch(a[1], low, high, n1);
break;

case 2 :
n2 = num2Dec(a[1], base);
low=findLeastRadix(a[0]);
high = (n2+1 > low+1) ? n2+1 : low+1;
radix = binarySearch(a[0], low, high, n2);
break;
}

if(-1 != radix){
printf("%ld\n", (long)radix);
}
else {
printf("Impossible");
}

return 0;
}