package leetCode_43;

import java.util.Arrays;

/**

* @author dimdark

*/

public class Solution {

public String multiply(String num1, String num2) {

if (num1 == null || num1.length() == 0 || num2 == null || num2.length() ==0) {

return null;

}

char[] cs1 = reverse(num1).toCharArray();

int len1 = cs1.length;

char[] cs2 = reverse(num2).toCharArray();

int len2 = cs2.length;

char[] cs = new char[len1 + len2];

Arrays.fill(cs, '0');

int addCarry, multiplyCarry;

int temp1, temp2;

for (int i = 0; i < len1; ++i) {

addCarry = 0;

multiplyCarry = 0;

for (int j = 0; j < len2; ++j) {

temp1 = (cs1[i] - '0') * (cs2[j] - '0') + multiplyCarry;

multiplyCarry = temp1 / 10;

temp2 = cs[i + j] - '0' + temp1 % 10 + addCarry;

addCarry = temp2 / 10;

cs[i + j] = (char) (temp2 % 10 + '0');

}

cs[i + len2] = (char) (cs[i + len2] + addCarry + multiplyCarry);

}

int end = cs.length - 1;

while (end > 0 && cs[end] == '0') {

--end;

}

return reverse(new String(cs, 0, end + 1));

}

private String reverse(String s) {

if (s == null) {

return null;

}

char[] cs = s.toCharArray();

int begin = 0;

int end = cs.length - 1;

while (begin < end) {

char temp = cs[begin];

cs[begin] = cs[end];

cs[end] = temp;

++begin;

--end;

}

return new String(cs);

}

}