LeetCode: Decode Ways

一开始dfs runtime exceed, 后来用dp想复杂了就看网上答案

class Solution {
public:
    int numDecodings(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int sum = 0;
        if (s.size() == 0) return sum;
        if (s.size() == 1 && s[0] >= '1' && s[0] <= '9') return 1;
        if (s[0] == '0') return 0;
        vector<int> ret(s.size());
        ret[0] = 1;
        for (int i = 1; i < s.size(); i++) {
            if (s.substr(i-1, 2) >= "10" && s.substr(i-1, 2) <= "26") {
                if (i > 1) ret[i] += ret[i-2];
                else ret[i] += 1;
            }
            if (s[i] >= '1' && s[i] <= '9') ret[i] += ret[i-1];
        }
        return ret[s.size()-1];
    }
};

 C#

public class Solution {
    public int NumDecodings(string s) {
        int sum = 0;
        if (s.Length == 0) return sum;
        if (s.Length == 1 && s[0] >= '1' && s[0] <= '9') return 1;
        if (s[0] == '0') return 0;
        int[] ans = new int[s.Length];
        ans[0] = 1;
        for (int i = 1; i < s.Length; i++) {
            if (string.Compare(s.Substring(i-1, 2), "10") >= 0 && string.Compare(s.Substring(i-1, 2), "26") <= 0) {
                if (i > 1) ans[i] += ans[i-2];
                else ans[i]++;
            }
            if (s[i] >= '1' && s[i] <= '9') ans[i] += ans[i-1];
        }
        return ans[s.Length-1];
    }
}