880. Decoded String at Index

An encoded string S is given. To find and write the decoded string to a tape, the encoded string is read one character at a time and the following steps are taken:

• If the character read is a letter, that letter is written onto the tape.
• If the character read is a digit (say d), the entire current tape is repeatedly written d-1 more times in total.

Now for some encoded string S, and an index K, find and return the K-th letter (1 indexed) in the decoded string.

class Solution {
public:
    string decodeAtIndex(string S, int K) {
        long len = 0;
        int i = 0;
        for(;len < K; i++) {
            if('0' <= S[i] && S[i] <= '9')
                len *= (S[i] & 0b1111);
            else
                ++len;
        }
        i--;
        for(; i >= 0; i--) {
            K %= len;
            if(!K && !('0' <= S[i] && S[i] <= '9'))
                return (string)"" + S[i];
            if('0' <= S[i] && S[i] <= '9')
                len /= (S[i] & 0b1111);
            else
                --len;
        }

        return "";
    }
};