91. Decode Ways

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

 

AC code: 　　space(n)

class Solution {
public:
    int numDecodings(string s) {
        if (s.length() == 0 || s == "0") return 0;
        int len = s.length();
        vector<int> dp(len+1, 0);
        dp[0] = 1;
        dp[1] = s[0] != '0' ? 1 : 0;
        for (int i = 2; i <= len; ++i) {
            int first = atoi(s.substr(i-1, 1).c_str());
            int second = atoi(s.substr(i-2, 2).c_str());
            if (first != 0)
                dp[i] += dp[i-1];
            if (second >= 10 && second <= 26)
                dp[i] += dp[i-2];
        }
        return dp[len];
    }
};

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Decode Ways.

improve code: 　　space(1) 

class Solution {
public:
    int numDecodings(string s) {
        if (s.length() == 0 || s == "0" || s[0] == '0') return 0;
        int c1 = 1;
        int c2 = 1;
        for (int i = 1; i < s.length(); ++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;
    }
};

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Decode Ways.