ACM-374 Big Mod with C++

/********************************/
/* This program is used to      */
/*    calculate                 */
/*    R = B^P mod M             */
/********************************/


#include <iostream>
#include <ctype.h>
#include <cstdio>
#include <math.h>

using namespace std;
long long M;
long long ans_power(long long a ,long long b);


int main(int argc, char* argv[])
{
 long long B,P;
 long long res;
 
 while(cin>>B>>P>>M)
 {
  if(M == 1)
  {
   res = 0;
  }
  else if(B == 0 && P == 0)   //0^0 = 1
  {
   res = 1;
  }
  else if(B == 0 && P != 0)  //0^n = 0
  {
   res = 0;
  }
  else if(B != 0 && P == 0)  //n^0 = 1
  {
   res = 1;
  }
  else
  {
   res = ans_power(B,P) % M;
  }
  cout<<res<<endl;
 }
 return 0;
}


long long ans_power(long long a ,long long b)
{
 if (b == 0) 
 {
  return 1;
 }
 else if ((b % 2) == 1) 
 {
  long long foo = ans_power(a, (b/2));
  return ((foo * foo * a) % M);
 }
 else
 {
  long long foo = ans_power(a, (b/2));
  return ((foo * foo) % M);
 }
}

本來用for迴圈做

也是吃了TLE

決定切開來mod這樣比較快

不然本來2的300次方

迴圈跑300次

切開之後變成超省時