[LeetCode] Decode Ways

int numDecodings(string s) 
{
	if (s.length() == 0 || s[0] == '0')
	{
		return 0;
	}
	else if (s.length() == 1)
	{
		return 1;
	}
	int numWithoutLastTwo = 1, numWithoutLastOne = 1, num = 0;
	for (int i = 1; i < s.length(); i++)
	{
		num = 0;
		if (s[i] != '0')
		{
			num += numWithoutLastOne;
		}
		if (s[i-1] == '1' || ((s[i-1] == '2') && (s[i] <= '6')))
		{
			num += numWithoutLastTwo;
		}
		numWithoutLastTwo = numWithoutLastOne;
		numWithoutLastOne = num;
	}
	return num;
}

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