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. http://leetcode.com/onlinejudge

leetcode_decode_ways.cpp

class Solution {
public:
    int numDecodings(string s) {
        if (s.empty())
            return 0;
        int n = s.length();
        int f[3] = {1, 1, 1};
        for (int i = 0; i < n; ++i) {
            f[i % 3] = 0;
            if (s[i] > '0')
                f[i % 3] += f[(i + 2) % 3];
            if (i != 0 && s[i - 1] > '0' && (s[i - 1] - '0') * 10 + s[i] - '0' <= 26)
                f[i % 3] += f[(i + 1) % 3];
        }
        return f[(n + 2) % 3];
    }
};