大数操作

这类问题有不使用运算符实现相乘，相加等

43. 字符串相乘

class Solution {
    public String multiply(String num1, String num2) {
        int len1 = num1.length();
        int len2 = num2.length();
        int[] res = new int[len1+len2];
        for(int i=len1-1;i>=0;i--){
            for(int j=len2-1;j>=0;j--){
                int x = num1.charAt(i)-'0';
                int y = num2.charAt(j)-'0';
                int pos1 = i + j;
                int pos2 = i + j + 1;
                int sum = x*y + 10*res[pos1] + res[pos2];
                res[pos2] = sum%10;
                res[pos1] = sum/10;
            }
        }

        StringBuilder sb = new StringBuilder();
        for(int i=0;i<res.length;i++){
            if(res[i] == 0&&sb.length() == 0){
                continue;
            }else{
                sb.append(res[i]);
            }
        }
        return sb.length()==0?"0":sb.toString();
    }
}