大整数乘法算法

刷笔试题的时候遇到了一个大整数乘法的问题，做法就是模拟手工算法，只要注意不能用long来存储，而要使用string。其中还有一些细节需要注意，例如进位，正负号等。

代码如下：

public static void main(String[] args) {

    Scanner in = new Scanner(System.in);

    while (in.hasNext()) {

        boolean s = true;

        String a = in.next();

        String b = in.next();

        if (a.charAt(0) == '-') {

            a = a.substring(1, a.length());

            s = !s;

        }

        if (b.charAt(0) == '-') {

            b = b.substring(1, b.length());

            s = !s;

        }

        String[] resList = new String[1000];

        for (int i = 0; i < 1000; i++) {

            resList[i] = "0";

        }

        int plus = 0;

        for (int i = b.length() - 1; i >= 0; i--) {

            String tRes = "";

            int now = Integer.parseInt(b.charAt(i) + "");

            for (int j = a.length() - 1; j >= 0; j--) {

                int now2 = Integer.parseInt(a.charAt(j) + "");

                int tmp = now * now2 + plus;

                if (tmp < 10) {

                    tRes = tRes + tmp;

                    plus = 0;

                } else {

                    tRes = tRes + (tmp % 10);

                    plus = tmp / 10;

                }

            }

            if (plus != 0) {

                tRes = tRes + plus;

                plus = 0;

            }

            boolean plus2 = false;

            for (int y = 0; y < tRes.length(); y++) {

                int tmp = Integer.parseInt(tRes.charAt(y) + "") + Integer.parseInt(resList[y + (b.length() - 1 - i)]) + (plus2 ? 1 : 0);

                if (tmp < 10) {

                    resList[y + (b.length() - 1 - i)] = tmp + "";

                    plus2 = false;

                } else {

                    resList[y + (b.length() - 1 - i)] = tmp - 10 + "";

                    plus2 = true;

                }

            }

            if (plus2) {

                resList[(b.length() - 1 - i) + tRes.length()] = Integer.parseInt(resList[(b.length() - 1 - i) + tRes.length()]) + "1";

                plus2 = false;

            }

        }

        boolean start = false;

        System.out.print(s ? "" : "-");

        for (int i = 999; i >= 0; i--) {

            if (!start && resList[i].equals("0")) {

                continue;

            }

            if (!start && !resList[i].equals("0")) {

                start = true;

                System.out.print(resList[i]);

            } else {

                System.out.print(resList[i]);

            }

        }

    }

}