LeetCode 91. Decode Ways

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class Solution {
public:
    int numDecodings(string s) {
        const int n = s.size();
        vector<int> dp(n + 1);
        dp[0] = 1;
        for (int i = 1; i <= n; ++i) {
            if (s[i - 1] != '0')
                dp[i] += dp[i - 1];
            if (i > 1 && s[i - 2] != '0' && ((s[i - 2] - '0') * 10 + (s[i - 1] - '0') <= 26))
                dp[i] += dp[i - 2];
        }
        return dp[n];
    }
};
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class Solution {
public:
    int numDecodings(string s) {
        const int n = s.size();
        // a = dp[i - 2], b = dp[i - 1], c = dp[i]
        int a = 0, b = 1, c;
        for (int i = 1; i <= n; ++i) {
            c = 0;
            if (s[i - 1] != '0')
                c += b;
            if (i > 1 && s[i - 2] != '0' && ((s[i - 2] - '0') * 10 + (s[i - 1] - '0') <= 26))
                c += a;
            tie(a, b) = {b, c};
        }
        return c;
    }
};