leetcode 91. Decode Ways

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

class Solution {
	int fibnacci_sum(int n)
	{
		if (n == 0)
			return 1;
		if (n == 1)
			return 1;
		if (n == 2)
			return 2;
		return fibnacci_sum(n - 1) + fibnacci_sum(n - 2);
	}
public:
	int numDecodings(string s) {
		if (s.empty())
			return 0;
		if (s.length() == 1)
		{
			if (s[0] != '0')
				return 1;
			return 0;
		}
		int k = 0;
		int nums = 1;
		while (k + 1 < s.length())
		{
			if (s[0] == '0')
				return 0;
			k = 0;
			while (k + 1 < s.length())
			{
				if (s[k] >= '3')
				{
					nums *= fibnacci_sum(k+1);
					s.erase(0, k + 1);
					k = 0;
					break;
				}
				if (s[k] == '0')
				{
					nums *= fibnacci_sum(k - 1);
					s.erase(0, k + 1);
					k = 0;
					break;
				}
				if (s[k] == '2'&&s[k + 1] > '6')
				{
					nums *= fibnacci_sum(k + 1);
					s.erase(0, k + 1);
					k = 0;
					break;
				}
				k++;
			}
		}
		if (s.length() > 1)
		{
			if (s[k] >= '3')
				nums *= fibnacci_sum(k+1);
			else if (s[k] == '0')
				nums *= fibnacci_sum(k - 1);
			else
				nums *= fibnacci_sum(s.length());
			return nums;
		}
		if (s.empty())
			return nums;
		if (s[0] == '0')
			return 0;
		return nums;
	}
};

accepted