add code of 13

Author: Li Li

top-interview-questions/13. Roman to Integer.cs

@@ -0,0 +1,23 @@
+public class Solution {
+    public int RomanToInt(string s) {
+        if (s == null || s.Length == 0) return 0;
+        Dictionary<char, int> dict = new Dictionary<char, int>() {
+            {'I', 1},
+            {'V', 5},
+            {'X', 10},
+            {'L', 50},
+            {'C', 100},
+            {'D', 500},
+            {'M', 1000},
+        };
+        int sum = dict[s[s.Length - 1]];
+        for (int i = s.Length - 2; i >= 0; i--) {
+            if (dict[s[i]] < dict[s[i + 1]]) {
+                sum -= dict[s[i]];
+            } else {
+                sum += dict[s[i]];
+            }
+        }
+        return sum;
+    }
+}