feat: added two sol

Author: Guan Cong

csharp/12.integer-to-roman.cs

@@ -0,0 +1,42 @@
+/*
+ * @lc app=leetcode id=12 lang=csharp
+ *
+ * [12] Integer to Roman
+ */
+
+// @lc code=start
+public class Solution {
+    public string IntToRoman(int num) {
+        var result = new StringBuilder();
+        var map = new Dictionary<int, string>
+        {
+            { 1000, "M" },
+            { 900, "CM" },
+            { 500, "D" },
+            { 400, "CD" },
+            { 100, "C" },
+            { 90, "XC" },
+            { 50, "L" },
+            { 40, "XL" },
+            { 10, "X" },
+            { 9, "IX" },
+            { 5, "V" },
+            { 4, "IV" },
+            { 1, "I" },
+        };
+        while (num > 0) {
+            foreach (var key in map) {
+                if (num >= key.Key) {
+                    result.Append(key.Value);
+                    num -= key.Key;
+                    break;
+                }
+            }
+        }
+
+        return result.ToString();
+
+    }
+}
+// @lc code=end
+

csharp/8.string-to-integer-atoi.cs

@@ -0,0 +1,28 @@
+/*
+ * @lc app=leetcode id=8 lang=csharp
+ *
+ * [8] String to Integer (atoi)
+ */
+
+// @lc code=start
+public class Solution {
+    public int MyAtoi(string s) {
+        int i = 0; // index
+        int sign = 1; // sign
+        int result = 0; 
+        if (s.Length == 0) return 0; // empty string
+        while (i < s.Length && s[i] == ' ') i++; // skip white spaces
+        if (i < s.Length && (s[i] == '+' || s[i] == '-')) { // check sign
+            sign = (s[i++] == '+') ? 1 : -1; // skip sign
+        }
+        while (i < s.Length && s[i] >= '0' && s[i] <= '9') { // check number
+            if (result > Int32.MaxValue / 10 || (result == Int32.MaxValue / 10 && s[i] - '0' > 7)) { // check overflow
+                return (sign == 1) ? Int32.MaxValue : Int32.MinValue; // return max or min
+            }
+            result = result * 10 + (s[i++] - '0'); // calculate result
+        }
+        return result * sign;
+    }
+}
+// @lc code=end
+