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void add (BigInt & res,const BigInt &a,const BigInt &b) {
//value of res is altered.
}
void sub(BigInt & res,const BigInt & a,const BigInt & b) {
//value of res is altered.
}
BigInt operator + (const BigInt & a,const BigInt & b) {
BigInt res;
res.digits.reserve(a.digits.size()+2);
if (b.negative==a.negative) {
if (b.digits.size()>a.digits.size()) {
add(res,b,a);
} else {
add(res,a,b);
}
} else if (a.negative) {
if (b>a) {
sub(res,b,a);
res.negative=false;
} else {
sub(res,a,b);
res.negative=true;
}
} else {
if (a>b) {
sub(res,a,b);
res.negative=false;
} else {
sub(res,b,a);
res.negative=true;
}
}
return res;
}
BigInt operator - (const BigInt & a,const BigInt & b) {
BigInt res;
res.digits.reserve(a.digits.size()+2);
if (!b.negative && !a.negative) {
if (a>b) {
sub(res,a,b);
} else {
sub(res,b,a);
res.negative=true;
}
} else if (b.negative && a.negative) {
if (b>a) {
sub(res,b,a);
} else {
sub(res,a,b);
res.negative=true;
}
} else if (b.negative) {
add(res,a,b);
} else {
add(res,a,b);
res.negative=true;
}
return res;
}
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