[3]43. Multiply Strings(Java)

注：此博客不再更新，所有最新文章将发表在个人独立博客limengting.site。分享技术，记录生活，欢迎大家关注

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2.

Note:

1、  The length of both num1 and num2 is < 110.
2、  Both num1 and num2 contains only digits 0-9.
3、  Both num1 and num2 does not contain any leading zero.
4、  You must not use any built-in BigInteger library or convert the inputs to integer directly.

class Solution {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] res = new int[m + n];
        
        // num1[i] * num2[j] -> res(i + j, i + j + 1)设为res(p1, p2)
        for (int i = m - 1; i >= 0 ; i --) {
            for (int j = n - 1; j >= 0; j --) {
                // oneMulOne一位乘以一位的积，sum为乘积与原来的低位相加之和
                int oneMulOne = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                int p1 = i + j, p2 = i + j + 1;
                int sum = oneMulOne + res[p2];
                
                res[p1] += sum / 10;
                res[p2] = sum % 10;   
            }
        }
        
        StringBuilder sb = new StringBuilder();
        for(int i : res) if(!(sb.length() == 0 && i == 0)) sb.append(i);
        return sb.length() == 0 ? "0" : sb.toString();
    }
}