LeetCode: Decode Ways

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

 1 class Solution {

 2 public:

 3     int numDecodings(string s) {

 4         // Start typing your C/C++ solution below

 5         // DO NOT write int main() function

 6         int sum = 0;

 7         if (s.size() == 0) return sum;

 8         if (s.size() == 1 && s[0] >= '1' && s[0] <= '9') return 1;

 9         if (s[0] == '0') return 0;

10         vector<int> ret(s.size());

11         ret[0] = 1;

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

13             if (s.substr(i-1, 2) >= "10" && s.substr(i-1, 2) <= "26") {

14                 if (i > 1) ret[i] += ret[i-2];

15                 else ret[i] += 1;

16             }

17             if (s[i] >= '1' && s[i] <= '9') ret[i] += ret[i-1];

18         }

19         return ret[s.size()-1];

20     }

21 };

 C#

 1 public class Solution {

 2     public int NumDecodings(string s) {

 3         int sum = 0;

 4         if (s.Length == 0) return sum;

 5         if (s.Length == 1 && s[0] >= '1' && s[0] <= '9') return 1;

 6         if (s[0] == '0') return 0;

 7         int[] ans = new int[s.Length];

 8         ans[0] = 1;

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

10             if (string.Compare(s.Substring(i-1, 2), "10") >= 0 && string.Compare(s.Substring(i-1, 2), "26") <= 0) {

11                 if (i > 1) ans[i] += ans[i-2];

12                 else ans[i]++;

13             }

14             if (s[i] >= '1' && s[i] <= '9') ans[i] += ans[i-1];

15         }

16         return ans[s.Length-1];

17     }

18 }

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