Grade-school multiplication with an integer array.
Use res[i+j+1] += digit1 * digit2 for all pairs.
Then propagate carries from right to left.
Finally join digits, skipping leading zeros.
Grade-school multiplication with an integer array.
Use res[i+j+1] += digit1 * digit2 for all pairs.
Then propagate carries from right to left.
Finally join digits, skipping leading zeros.