字符串系列一＞字符串相乘

class Solution {
    public String multiply(String num1, String num2) {
        //1.无进相乘后相加。2.处理进位
        char[] s1 = new StringBuffer(num1).reverse().toString().toCharArray();
        char[] s2 = new StringBuffer(num2).reverse().toString().toCharArray();
        int n = s1.length;
        int m = s2.length;

        int[] tmp = new int[m+n-1];

        for(int i = 0; i < n; i++)
            for(int j = 0; j < m; j++)
                tmp[i+j] += (s1[i]-'0') * (s2[j]-'0');

        //相加
        StringBuffer ret = new StringBuffer();
        int cur = 0,t = 0;
        while(cur < m+n-1 || t != 0){
            if(cur < m+n-1) t += tmp[cur++]; 
            ret.append((char)((char)t % 10 + '0'));
            t /= 10;//进位
        }

        //2.处理前导0: (注意算出来是逆序的)
        while(ret.length() > 1 && ret.charAt(ret.length() - 1) == '0'){
            ret.deleteCharAt((ret.length()-1));
        }

        return ret.reverse().toString(); 
    }
}