add solution for 013 in js

Author: limbowandering

solution/013.Roman to Integer/Solution.js

@@ -0,0 +1,20 @@
+const romanToInt = function(s){
+  const map = {
+    'I' : 1,
+    'V' : 5,
+    'X' : 10,
+    'L' : 50,
+    'C' : 100,
+    'D' : 500,
+    'M' : 1000
+  };
+  let sum = 0;
+  for(let i = 0; i < s.length;i++){
+    if(map[s[i]] < map[s[i+1]]){
+      sum-=map[s[i]];
+    }else{
+      sum+=map[s[i]];
+    }
+  }
+  return sum;
+};