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MultiplyStrings.java

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public class MultiplyStrings {

public String multiply(String num1, String num2) {

if(num1 == null || num2 == null )

return null;

if (num1.equals("0") || num2.equals("0"))

return "0";

String result = "";

//for each digits in num2 multiply each digits in num1. each digits count with all digits in num1, save the temp. then add up all such temp.

for(int i= num2.length()-1;i>=0;i--){

if(num2.charAt(i) == '0')

continue;

String temp = "";

int carry = 0;

for(int j = num1.length()-1;j>=0;j--){

int digit = Character.digit(num2.charAt(i), 10)*Character.digit(num1.charAt(j), 10)+carry;

carry = digit/10;

temp = (char)('0'+ digit%10) + temp;

}

if (carry != 0)

temp = (char)('0'+carry) + temp;

//add 0 at the end, to match up.

int count = num2.length()-1-i;

while(count>0){

temp = temp+'0';

count--;

}

//add the current temp to result

result = add(temp,result);

}

return result;

}

private String add(String s1, String s2) {

// TODO Auto-generated method stub

int i = s1.length()-1;

int j = s2.length()-1;

int carry = 0;

String result = "";

while(i>=0 && j>=0){

int digit = Character.digit(s1.charAt(i), 10)+ Character.digit(s2.charAt(j), 10)+carry;

carry = digit/10;

result = (char)('0'+digit%10) + result;

i--;

j--;

}

while (i>=0){

int digit = Character.digit(s1.charAt(i), 10) + carry;

carry = digit/10;

result = (char)('0'+digit%10) + result;

i--;

}

while (j>=0){

int digit = Character.digit(s2.charAt(j), 10) + carry;

carry = digit/10;

result = (char)('0'+digit%10) + result;

j--;

}

if (carry != 0)

result = (char)('0'+carry) + result;

return result;

}

/**

* @param args

*/

public static void main(String[] args) {

// TODO Auto-generated method stub

String s1 = "498828660196";

String s2 = "840477629533";

MultiplyStrings so = new MultiplyStrings();

System.out.println(so.multiply(s1, s2));

}

}