字符串相乘

按位依次相乘，相加再进位

class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0") || num2.equals("0")){
            return "0";
        }
        int m = num1.length();
        int n = num2.length();
        int[] mn = new int[m+n];
        for(int i = m-1;i >= 0;i--){
            int x = num1.charAt(i) - '0';
            for(int j = n-1;j >= 0;j--){
                int y = num2.charAt(j) - '0';
                mn[i+j+1] += x * y; 
            }
        }
        for(int i = m + n - 1;i > 0;i--){
            mn[i-1] += mn[i]/10;
            mn[i] = mn[i] % 10;
        }
        StringBuffer result = new StringBuffer();
        int index = mn[0] == 0 ? 1:0;
        while(index < m+n){
            result.append(mn[index]);
            index++;
        }
        return result.toString();
    }
}