added combinationMath solution

combinationMath.js

@@ -0,0 +1,55 @@
+/////////////////////////////////////////////////////////////
+// Combination Math
+// -------------------------------
+// Given a initial string of numbers, and a target, find the list of combinations
+// of operators and numbers that equal the target.
+//
+// ex: comboSum('1234', 10) =>
+//  ["1+2+3+4", "1*2*3+4"]
+//
+// extra credit:
+// Don't use eval
+//
+/////////////////////////////////////////////////////////////
+
+var mathMap = {
+    '+': function(a, b) {
+        return a + b
+    },
+    '*': function(a, b) {
+        return a * b
+    }
+}
+
+var evaluateInfix = function(str, operators) {
+    return str.split('')
+        .map(function(i) {
+            return parseInt(i)
+        })
+        .reduce(function(prev, cur, index) {
+            var stringOperator = operators[index - 1]
+            return mathMap[stringOperator](prev, cur)
+        })
+}
+
+var combinationMath = function(str, target, operators, result) {
+    if (operators.length === str.length - 1) {
+        if (evaluateInfix(str, operators) === target) {
+            return str.split('')
+                .map(function(value, index) {
+                    if (operators[index]) {
+                        return value += operators[index]
+                    } else {
+                        return value
+                    }
+                }).join('')
+        }
+	    return []
+    }
+    // all combinations of operators
+    return result.concat(combinationMath(str, target, operators.concat('+'), result) , combinationMath(str, target, operators.concat('*'), result))
+}
+
+console.log(combinationMath('12345', 15, [], []))
+
+module.exports = combinationMath;