Multiply Strings And Add Strings

43. Multiply Strings

图解

string multiply(string num1, string num2) {
    string sum(num1.size() + num2.size(), '0');
    
    for (int i = num1.size() - 1; 0 <= i; --i) {
        int carry = 0;
        for (int j = num2.size() - 1; 0 <= j; --j) {
            int tmp = (sum[i + j + 1] - '0') + (num1[i] - '0') * (num2[j] - '0') + carry;
            sum[i + j + 1] = tmp % 10 + '0';
            carry = tmp / 10;
        }
        sum[i] += carry;
    }
    
    size_t startpos = sum.find_first_not_of("0");
    if (string::npos != startpos) {
        return sum.substr(startpos);
    }
    return "0";
}

415. Add Strings

class Solution {
public:
string addStrings(string num1, string num2) {
    int i = num1.size() - 1;
    int j = num2.size() - 1;
    int carry = 0;
    string res = "";
    while(i>=0 || j>=0 || carry){
        long sum = 0;
        if(i >= 0){sum += (num1[i] - '0');i--;}
        if(j >= 0){sum += (num2[j] - '0');j--;}
        sum += carry; 
        carry = sum / 10;
        sum = sum % 10;
        res =  res + to_string(sum);
    }
    reverse(res.begin(), res.end());
    return res;
}
};