w3y · w3y · Aug 8, 2019 · Aug 8, 2019
diff --git a/src/RomantoInteger.java b/src/RomantoInteger.java
@@ -0,0 +1,75 @@
+/*
+Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.
+
+Symbol       Value
+I             1
+V             5
+X             10
+L             50
+C             100
+D             500
+M             1000
+
+For example, two is written as II in Roman numeral, just two one's added together.
+Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.
+
+Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII.
+Instead, the number four is written as IV. Because the one is before the five we subtract it making four.
+The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:
+
+    I can be placed before V (5) and X (10) to make 4 and 9.
+    X can be placed before L (50) and C (100) to make 40 and 90.
+    C can be placed before D (500) and M (1000) to make 400 and 900.
+
+Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.
+ */
+public class RomantoInteger {
+
+    public int romanToInt(String s) {
+        int result = 0;
+        char[] sArr = s.toCharArray();
+        int[] digits = buildDigits();
+        int sLen = sArr.length;
+        for (int i = 0; i < sLen; i++) {
+            int next = i + 1;
+            char c = sArr[i];
+            if (next >= sLen) {
+                result += digits[c - 'A'];
+            } else {
+                char n = sArr[next];
+                boolean isComplicated =
+                        (c == 'I' && (n == 'V' || n == 'X')) ||
+                                (c == 'X' && (n == 'L' || n == 'C')) ||
+                                (c == 'C' && (n == 'D' || n == 'M'));
+
+                if (isComplicated) {
+                    result += digits[n - 'A'] - digits[c - 'A'];
+                    i = next;
+                } else {
+                    result += digits[c - 'A'];
+                }
+            }
+        }
+        return result;
+    }
+
+    public int[] buildDigits() {
+        int[] digits = new int[24];
+        digits['I' - 'A'] = 1;
+        digits['V' - 'A'] = 5;
+        digits['X' - 'A'] = 10;
+        digits['L' - 'A'] = 50;
+        digits['C' - 'A'] = 100;
+        digits['D' - 'A'] = 500;
+        digits['M' - 'A'] = 1000;
+        return digits;
+    }
+
+    public static void main(String[] args) {
+        RomantoInteger romantoInteger = new RomantoInteger();
+        System.out.println(romantoInteger.romanToInt("IV"));
+        System.out.println(romantoInteger.romanToInt("VI"));
+        System.out.println(romantoInteger.romanToInt("VIII"));
+        System.out.println(romantoInteger.romanToInt("XVIII"));
+    }
+}