leetcode -- 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.

大整数乘法，一位一位往上乘，注意进位的处理即可。此外，注意0的处理

public class Solution {
    public String multiply(String num1, String num2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int l1 = num1.length();
        int l2 = num2.length();
        
        int[] num = new int[l1 + l2 + 1];
        for(int i = 0; i <= l2 - 1; i ++){
            int carry = 0;
            int a = num2.charAt(l2 - 1 - i) - '0';
            for(int j = 0; j <= l1 - 1; j ++){
                int b = num1.charAt(l1 - 1 - j) - '0';
                num[i + j] += a * b + carry;
                carry = num[i + j] / 10;
                num[i + j] = num[i + j] % 10;
            }
            num[i + l1] = carry;
        }
        
        int i = num.length - 1;
        while(i > 0 && num[i] == 0){
            i--;
        }
        
        StringBuilder sb = new StringBuilder();
        while(i >= 0){
            sb.append(num[i--]);
        }
        return sb.toString();
    }
}

 

ref:http://gongxuns.blogspot.com/2013/01/leetcode-multiply-strings.html