题目

decode-ways/

算法

* DP

* O（1）的算法

代码

* DP

class Solution {

public:

    int numDecodings(string s) {

        if (s.empty() || (s.size() > 1 && s[0] == '0')) return 0;

        vector<int> dp(s.size() + 1, 0);

        dp[0] = 1;

        for (int i = 1; i < dp.size(); ++i) {

            dp[i] = (s[i - 1] == '0') ? 0 : dp[i - 1];

            if (i > 1 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6'))) {

                dp[i] += dp[i - 2];

            }

        }

        return dp.back();

    }

};

* o(1)的算法

class Solution {

public:

    int numDecodings(string s) {

        if (s.empty() || s.front() == '0') return 0;

        int c1 = 1, c2 = 1;

        for (int i = 1; i < s.size(); ++i) {

            if (s[i] == '0') c1 = 0;

            if (s[i - 1] == '1' || (s[i - 1] == '2' && s[i] <= '6')) {

                c1 = c1 + c2;

                c2 = c1 - c2;

            } else {

                c2 = c1;

            }

        }

        return c1;

    }

};