[Leetcode] Decode Ways *

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class Solution {public:    int numDecodings(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int total = 0;                Decode(s, 0, s.size(), total);        return total;    }        void Decode(string& s, int idx, int length, int& total)    {        if (length == 0)            return;                if (idx == length - 1)        {                if (s[idx] != '0')                ++total;            return;        }                if (s[idx] != '0')        {            Decode(s, idx + 1, length, total);        }                if (s[idx] == '1' || (s[idx] == '2' && s[idx + 1] <= '6'))        {            if (idx + 1 == length - 1)            {                ++total;                return;            }            else            {                Decode(s, idx + 2, length, total);            }        }            }};

// DPclass Solution {public:    int numDecodings(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if (s.size() == 0) return 0;                vector<int> arr(s.size() + 2, 1);                for (int i = s.size() - 1; i >= 0; --i)        {            if (s[i] == '0')                arr[i] = 0;            else                arr[i] = arr[i + 1];                            if (i + 1 < s.size() && (s[i] == '1' || (s[i] == '2' && s[i + 1] <= '6')))                arr[i] += arr[i + 2];        }                return arr[0];    }};