Leetcode: 43. Multiply Strings

题目难度中等，想到分别遍历num1、num2中的数值，但是具体细节没有理清。参考：https://discuss.leetcode.com/topic/30508/easiest-java-solution-with-graph-explanation?page=1

public class TwoSumbin {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] pos = new int[m + n];
        for (int i = m - 1; i >= 0; i--) {
            for (int j = n - 1; j >= 0; j--) {
                int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                int p1 = i + j; // 十位数
                int p2 = i + j + 1; // 个位数
                int sum = mul + pos[p2];
                pos[p1] += sum / 10; // 开始时是在3，下一个循环pos[3]就是个位数，考虑到了进位 //在篇左侧的数字
                pos[p2] = sum % 10; // 偏右
            }
        }
        StringBuilder sb = new StringBuilder();
        for (int p : pos)
            if (!(sb.length() == 0 && p == 0))
                sb.append(p);
        return sb.length() == 0 ? "0" : sb.toString();

    }
}