【Multiply Strings】cpp

题目：

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

代码：

class Solution {
public:
string multiply(string num1, string num2) {
const int n1 = num1.size(), n2 = num2.size();
if ( num1=="0" || num2=="0") return "0";
vector<char> ret;
for ( int i=n1-1; i>=0; --i )
{
vector<char> local(1,'0');
int v = 0, carry = 0, curr = 0;
for ( int j=n2-1; j>=0; --j )
{
v= (num1[i]-'0')*(num2[j]-'0');
carry = v/10;
curr = v%10;
carry += ((local[0]-'0')+curr)/10;
curr = ((local[0]-'0')+curr)%10;
local[0] = curr+'0';
local.insert(local.begin(), carry+'0');
}
if (local[0]=='0') local.erase(local.begin());
// cout << string(local.begin(),local.end()) << endl;
// add zeros
for ( int z=n1-1; z>i; --z) local.push_back('0');
// cout << string(local.begin(),local.end()) << endl;
carry = 0, curr = 0;
for ( int r=ret.size()-1, l=local.size()-1; r>=0 || l>=0; --r,--l )
{
if ( r>=0 && l>=0 )
{
curr = (carry+(ret[r]-'0')+(local[l]-'0'))%10;
carry = (carry+(ret[r]-'0')+(local[l]-'0'))/10;
local[l] = curr+'0';
}
else
{
curr = (carry+(local[l]-'0'))%10;
carry = (carry+(local[l]-'0'))/10;
local[l] = curr+'0';
}
}
if (carry!=0) { local.insert(local.begin(), carry+'0'); }
ret = local;
}
return string(ret.begin(),ret.end());
}
};

tips：

就是一些字符串操作的细节，考虑进位，0这类的细节。

======================================

第二次过这道题，憋了好久。主要是有个思维误区，认为tmp比ret最多多一位；但是，比如52*4这样的例子，2*4=8 50*4=200就不止一位了。

解决了这个误区，代码AC了。

class Solution {
public:
string multiply(string num1, string num2) {
if ( num1=="0" || num2=="0" ) return "0";
vector<char> ret(num2.size(),'0');
for ( int i=num1.size()-1; i>=0; --i )
{
// mupltiply ret
vector<int> tmp;
int val = 0, carry = 0;
for ( int j=num2.size()-1; j>=0; --j )
{
val = (num2[j]-'0')*(num1[i]-'0')+carry;
tmp.insert(tmp.begin(), val%10);
carry = val/10;
}
if ( carry>0 ) tmp.insert(tmp.begin(), carry);
for ( int k=0; k<num1.size()-1-i; ++k ) tmp.push_back(0);
// merge tmp & ret
while ( tmp.size()>ret.size() ) ret.insert(ret.begin(),'0');
val = 0;
carry = 0;
for ( int m=tmp.size()-1; m>=0; --m )
{
val = (ret[m]-'0') + tmp[m] + carry;
ret[m] = val%10+'0';
carry = val/10;
}
if ( carry>0 ) ret.insert(ret.begin(), carry+'0');
}
return string(ret.begin(), ret.end());
}
};