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.

Solution:

1 翻转string

2 建立数组，双层循环遍历两个string，把单位的乘积累加到数组相应的位置

3 处理进位并输出

4 注意前导零的corner case

public class Solution {
    public String multiply(String num1, String num2) {
        //reverse the string first
        String n1 = new StringBuilder(num1).reverse().toString();
        String n2 = new StringBuilder(num2).reverse().toString();
        
        //res is for storing the result of multiply
        int[] res = new int[n1.length() + n2.length()];
        
        for(int i = 0; i < n1.length(); i++) {
            for(int j = 0; j < n2.length(); j++) {
                //count the result at right place
                res[i + j] += (n1.charAt(i) - '0') * (n2.charAt(j) - '0');
            }
        }
        
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < res.length; i++) {
            int digit = res[i] % 10;  //for current position
            int carry = res[i] / 10;  //for carry
            if(i + 1 < res.length) {
                res[i + 1] += carry;
            }
            sb.insert(0, digit);  //prepend
        }
        
        while(sb.charAt(0) == '0' && sb.length() > 1) {
            sb.deleteCharAt(0);  //delete 0 at ahead
        } 
        
        return sb.toString();
    }
}