Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2.
Note:
The length of both num1 and num2 is < 110.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.
题意：这是一道大数乘法问题，这类算法可能一开始，我们真的想不到，所以我在网上找到算法之后，在纸上画了画感觉就好很多了，大家也可以根据我的例子在纸上画一画。
大数乘法的例子
java代码：
public String multiply(String num1, String num2) {
num1 = new StringBuilder(num1).reverse().toString();
num2 = new StringBuilder(num2).reverse().toString();
int[] d = new int[num1.length() + num2.length()];
for (int i = 0; i < num1.length(); i++) {
int a = num1.charAt(i) - '0';
for (int j = 0; j < num2.length(); j++) {
int b = num2.charAt(j) - '0';
d[i + j] += a * b;
}
}
StringBuilder sb = new StringBuilder();
for (int i = 0; i < d.length; i++) {
int digit = d[i] % 10;
int carry = d[i] / 10;
sb.insert(0, digit);
if (i < d.length - 1)
d[i + 1] += carry;
}
// trim starting zeros
while (sb.length() > 0 && sb.charAt(0) == '0') {
sb.deleteCharAt(0);
}
return sb.length() == 0 ? "0" : sb.toString();
}
