CODE 90. Multiply Strings

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Given two numbers represented as strings, return multiplication of the numbers as a string.
Note: The numbers can be arbitrarily large and are non-negative.

public String multiply(String num1, String num2) {
	// Note: The Solution object is instantiated only once and is reused by
	// each test case.
	if ("0".equals(num1) || "0".equals(num2)) {
		return "0";
	}
	char[] num1s = num1.toCharArray();
	char[] num2s = num2.toCharArray();
	List<Character> result = new ArrayList<Character>();

	for (int i = num2s.length - 1; i >= 0; i--) {
		int num = 0;
		ArrayList<Character> line = new ArrayList<Character>();
		for (int j = num1s.length - 1; j >= 0; j--) {
			int tmp = Character.digit(num1s[j], 10)
					* Character.digit(num2s[i], 10) + num;
			int ge = tmp % 10;
			num = tmp / 10;
			line.add(0, Character.forDigit(ge, 10));
		}
		if (num > 0) {
			line.add(0, Character.forDigit(num, 10));
		}
		if (num2s.length - 1 == i) {
			result.addAll(line);
		} else {
			int nu = 0;
			int n = 0;
			int m = 0;
			for (m = result.size() - 1 - (num2s.length - 1 - i), n = line
					.size() - 1; m >= 0 && n >= 0; m--, n--) {
				int tmp = Character.digit(result.get(m), 10)
						+ Character.digit(line.get(n), 10) + nu;
				int ge = tmp % 10;
				result.set(m, Character.forDigit(ge, 10));
				nu = tmp / 10;
			}
			if (n >= 0 || nu > 0) {
				for (; n >= 0; n--) {
					int tmp = Character.digit(line.get(n), 10) + nu;
					int ge = tmp % 10;
					result.add(0, Character.forDigit(ge, 10));
					nu = tmp / 10;
				}
				if (nu > 0) {
					result.add(0, Character.forDigit(nu, 10));
				}
			}
		}
	}
	StringBuilder sb = new StringBuilder();
	for (int i = 0; i < result.size(); i++) {
		sb.append(result.get(i));
	}
	return sb.toString();
}