Merge pull request #16 from vaskoz/day7

Day7

README.md

@@ -15,4 +15,5 @@ problems from
 * [Day 4](https://github.com/vaskoz/dailycodingproblem-go/issues/5)
 * [Day 5](https://github.com/vaskoz/dailycodingproblem-go/issues/9)
 * [Day 5 merge k sorted lists](https://github.com/vaskoz/dailycodingproblem-go/issues/11)
-
+* [Day 6](https://github.com/vaskoz/dailycodingproblem-go/issues/13)
+* [Day 7](https://github.com/vaskoz/dailycodingproblem-go/issues/15)

day7/problem.go

@@ -0,0 +1,17 @@
+package day7
+
+// NumberOfDecodings calculates the number of possible decodings where
+// letters are encoding as a=1, b=2 ... z=26
+func NumberOfDecodings(str string) int {
+	if length := len(str); length == 0 || length == 1 {
+		return 1
+	}
+	var count int
+	if last := str[len(str)-1]; last > '0' && last <= '9' {
+		count = NumberOfDecodings(str[:len(str)-1])
+	}
+	if second := str[len(str)-2]; second == '1' || second == '2' && str[len(str)-1] < '7' {
+		count += NumberOfDecodings(str[:len(str)-2])
+	}
+	return count
+}

day7/problem_test.go

@@ -0,0 +1,29 @@
+package day7
+
+import "testing"
+
+var testcases = []struct {
+	input    string
+	expected int
+}{
+	{"111", 3},
+	{"1111", 5},
+	{"2626", 4},
+	{"1234", 3},
+}
+
+func TestNumberOfDecodings(t *testing.T) {
+	for _, tc := range testcases {
+		if result := NumberOfDecodings(tc.input); result != tc.expected {
+			t.Errorf("For input %v, expected %v but got %v", tc.input, tc.expected, result)
+		}
+	}
+}
+
+func BenchmarkNumberOfDecodings(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			NumberOfDecodings(tc.input)
+		}
+	}
+}