Merge pull request #1151 from Sagar-Wadhwa-726/contributing_branch

Added roman to integer in java

Author: akgmage

Hash Table/roman_to_integer.java

@@ -0,0 +1,42 @@
+// AUTHOR : Sagar Wadhwa
+class Solution {
+    public int romanToInt(String s) {
+        // Hashmap in java which stores key value pairs. Each key is unique and no key is repeated 
+        HashMap<Character,Integer> map = new HashMap<>();
+
+        // Defining the basic rules of mapping from roman numeral to integer numbers.
+        map.put('I',1);
+        map.put('V',5);
+        map.put('X',10);
+        map.put('L',50);
+        map.put('C',100);
+        map.put('D',500);
+        map.put('M',1000);
+
+        //extract the last character of the string, and find it in the map, store this keys value in the 
+        // variable named "res"
+        int res = map.get(s.charAt(s.length()-1));
+
+        //iterate over the string from the second last character till the first character. Idea to solve
+        // this problem is that in the loop, for every character we check whether the value of the roman 
+        // numeral at the i+1 th index is greater than or less than the value of the roman numeral at i th 
+        // index. If the value of the roman numeral at the i th index is less than the value of the 
+        // roman numeral at i+1 th index, then obviously that means we have to subtract the value of the 
+        // roman numeral at the i th index from the result calculated so far. Else we have to add the value 
+        // of the roman numeral at the i th index to the result calculated so far. Example: V means five,
+        // but if it is IV, then here the value of the roman numeral I is less than the value of the roman 
+        // numeral V, so that means subtract I from V (4). If the string is VI, means 6, then here the value of 
+        // V is greater than the value of I, so add I in V, that is 6. 
+        // Doing so in an iterative fashion and storing the result and then adding or subtracting values 
+        // from it accordingly will give us our final result.
+        for(int i=s.length()-2;i>=0;i--){
+
+            if(map.get(s.charAt(i)) < map.get(s.charAt(i+1))){
+                res -= map.get(s.charAt(i));
+            }else{
+                res += map.get(s.charAt(i));
+            }
+        }
+        return res;
+    }
+}