//大數相減 
//調用形式：N.Sub(A)，返回值：N-A 
//若兩大數符號相同，其值相減，否則改變參數符號再調用大數相加函數 
/******************************************************************/ 
例如： 
A B C 
- D E 
-------------- 
= F G H 

其中，若C>=E，則H=C-E，carry(借位標志)=0 
若C<E，則H=C-E+0x100000000，carry=1 

若B-carry>=D，則G=B-carry-D，carry=0 
若B-carry<D，則G=B-carry-D+0x10000000，carry=1 

若carry=0，則F=A 
若carry=1，A>1，則F=A-1 
若carry=1，A=1，則F=0 
/*****************************************************************/ 
CBigInt CBigInt::Sub(CBigInt& A) 
{ 
CBigInt X; 
if(m_nSign==A.m_nSign) 
{ 
X.Mov(*this); 
int cmp=X.Cmp(A); 
if(cmp==0){X.Mov(0);return X;} 
int len,carry=0; 
unsigned __int64 num; 
unsigned long *s,*d; 
if(cmp>0) 
{ 
s=X.m_ulvalue; 
d=A.m_ulvalue; 
len=X.m_nLength; 
} 
if(cmp<0) 
{ 
s=A.m_ulvalue; 
d=X.m_ulvalue; 
len=A.m_nLength; 
X.m_nSign=1-X.m_nSign; 
} 
for(int i=0;i<len;i++) 
{ 
if((s[i]-carry)>=d[i]) 
{ 
X.m_ulvalue[i]=s[i]-carry-d[i]; 
carry=0; 
} 
else 
{ 
num=0x100000000+s[i]; 
X.m_ulvalue[i]=(unsigned long)(num-carry-d[i]); 
carry=1; 
} 
} 
while(X.m_ulvalue[len-1]==0)len--; 
X.m_nLength=len; 
return X; 
} 
else{X.Mov(A);X.m_nSign=1-X.m_nSign;return Add(X);} 
}