Merge d214472 into 3955911

README.md

@@ -82,3 +82,4 @@ problems from
 * [Day 70](https://github.com/vaskoz/dailycodingproblem-go/issues/149)
 * [Day 71](https://github.com/vaskoz/dailycodingproblem-go/issues/151)
 * [Day 72](https://github.com/vaskoz/dailycodingproblem-go/issues/153)
+* [Day 73](https://github.com/vaskoz/dailycodingproblem-go/issues/155)

day73/problem.go

@@ -0,0 +1,20 @@
+package day73
+
+// SinglyLL represents a node in a singly linked list.
+type SinglyLL struct {
+	Value interface{}
+	Next  *SinglyLL
+}
+
+// ReverseSinglyLinkedList reverses a singly linked list in O(1) space
+// and O(N) time.
+func ReverseSinglyLinkedList(head *SinglyLL) *SinglyLL {
+	var reversed *SinglyLL
+	for head != nil {
+		next := head.Next
+		head.Next = reversed
+		reversed = head
+		head = next
+	}
+	return reversed
+}

day73/problem_test.go

@@ -0,0 +1,50 @@
+package day73
+
+import "testing"
+
+var testcases = []struct {
+	input []interface{}
+}{
+	{[]interface{}{"first", 2, "third", 4, "fifth", 6}},
+	{[]interface{}{"foo", "bar", "baz"}},
+	{[]interface{}{1, 2, 3}},
+}
+
+func createSinglyLL(data []interface{}) *SinglyLL {
+	head := &SinglyLL{}
+	current := head
+	for _, value := range data {
+		current.Next = &SinglyLL{Value: value, Next: nil}
+		current = current.Next
+	}
+	return head.Next
+}
+
+func wasReversed(head *SinglyLL, data []interface{}) bool {
+	for i := range data {
+		if head == nil || head.Value != data[len(data)-1-i] {
+			return false
+		}
+		head = head.Next
+	}
+	return head == nil
+}
+
+func TestReverseSinglyLinkedList(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		sll := createSinglyLL(tc.input)
+		if reversed := ReverseSinglyLinkedList(sll); !wasReversed(reversed, tc.input) {
+			t.Errorf("This list was not reversed correctly: %v", tc.input)
+		}
+	}
+}
+
+func BenchmarkReverseSinglyLinkedList(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			sll := createSinglyLL(tc.input)
+			ReverseSinglyLinkedList(sll)
+		}
+	}
+}