91. 解码方法

class Solution {
public:
    int numDecodings(string s) {
    int n = s.size();
    vector<int> dp(n, 0);
    if (s[0] == '0')
        return 0;
    dp[0] = 1;
    for (int i = 1; i < n; i++) {
        string str = "";
        str += s[i-1];
        str += s[i];
        int num = stoi(str);
        cout << i << endl;
        if (num % 10 == 0 && (num < 1 || num > 26))
            return 0;
        if (num >= 11 && num <= 26 && num % 10 != 0) {
            if (i - 2 >= 0)
                dp[i] = dp[i-1] + dp[i-2];
            else
                dp[i] = dp[i-1] + 1;
        } else if (num >= 1 && num <= 26 && num % 10 == 0) {
            if (i - 2 >= 0)
                dp[i] = dp[i-2];
            else
                dp[i] = 1;
        } else {
            dp[i] = dp[i-1];
        }
    }
        return dp[n-1];
    }
};
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