43. Multiply Strings

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.

Converting the input string to integer is NOT allowed.

You should NOT use internal library such as BigInteger.

num1[i] * num2[j]` will be placed at indices `[i + j`, `i + j + 1]`  //参考大神的code https://discuss.leetcode.com/topic/30508/easiest-java-solution-with-graph-explanation

public class Solution {
public String multiply(String num1, String num2) {
int m = num1.length();
int n = num2.length();
int[] pos = new int[m+n];
for(int i = m -1 ; i>= 0; i--){
for(int j = n-1; j >= 0; j--){
int pos1 = i + j;
int pos2 = i+j+1;
int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
int sum = mul + pos[pos2];
pos[pos2] = sum % 10;
pos[pos1] += sum /10;
}
}

StringBuilder sb = new StringBuilder();
for(int p : pos) {
if(!(sb.toString().length() == 0 && p == 0))  //"0" * "0" = "00"
sb.append(p);
}
return sb.toString().length() == 0? "0":sb.toString() ;
}
}