class Solution {
public:
    int numDecodings(string s) {
        int n = s.length();
        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];
    }
};

空间压缩

class Solution {
public:
    int numDecodings(string s) {
        int n = s.length();
        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;
            }
            a = b;
            b = c;
        }
        return c;
    }
};