LeetCode/Multiply_String.java at master · Mengqi/LeetCode · GitHub

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publicclassSolution {
publicStringmultiply(Stringnum1, Stringnum2) {
Stringresult = "0";

if (num1.equals("0") || (num2.equals("0"))) {
returnresult;
 }

intlength = num2.length();
Stringbase = "";
for (inti = length-1; i >= 0; i--) {
chardigit = num2.charAt(i);
intnumDigit = digit - '0';
Stringres = multiplyByDigit(num1, numDigit) + base;
result = add(res, result);
base = base + '0';
 }
returnresult;
 }

privateStringmultiplyByDigit(Stringnum, intdigit) {
if (digit == 0) {
return"0";
 }

Stringresult = "";
intlength = num.length();
intcarry = 0;
for (inti = length-1; i >= 0; i--) {
intnumber = num.charAt(i) - '0';
intres = number * digit + carry;
carry = res / 10;
res = res % 10;
result = res + result;
 }
if (carry != 0) {
result = carry + result;
 }
returnresult;
 }

privateStringadd(Stringnum1, Stringnum2) {
intcarry = 0;
intpos1 = num1.length() - 1;
intpos2 = num2.length() - 1;
Stringresult = "";
while ((pos1 >= 0) && (pos2 >= 0)) {
intdigit1 = num1.charAt(pos1) - '0';
intdigit2 = num2.charAt(pos2) - '0';
intresDigit = digit1 + digit2 + carry;
carry = resDigit / 10;
resDigit = resDigit % 10;
result = resDigit + result;
pos1--;
pos2--;
 }

while (pos1 >= 0) {
intdigit1 = num1.charAt(pos1) - '0';
intresDigit = digit1 + carry;
carry = resDigit / 10;
resDigit = resDigit % 10;
result = resDigit + result;
pos1--;
 }

while (pos2 >= 0) {
intdigit2 = num2.charAt(pos2) - '0';
intresDigit = digit2 + carry;
carry = resDigit / 10;
resDigit = resDigit % 10;
result = resDigit + result;
pos2--;
 }

if (carry != 0) {
result = carry + result;
 }

returnresult;
 }
}