reverse a linked list

interviewcake/reverse_linked_list_test.go

@@ -0,0 +1,67 @@
+// Problem:
+// Reverse a linked list in-place.
+//
+// Solution:
+// Iterate through the list and point each node's next pointer to the previous
+// item.
+//
+// Cost:
+// O(n) time and O(1) space.
+
+package interviewcake
+
+import (
+	"testing"
+
+	"github.com/hoanhan101/algo/common"
+)
+
+func TestReverseLinkedList(t *testing.T) {
+	// define tests input.
+	t1 := &LinkedList{}
+
+	t2 := &LinkedList{1, nil}
+
+	t3 := &LinkedList{1, nil}
+	t3.next = &LinkedList{2, nil}
+
+	t4 := &LinkedList{1, nil}
+	t4.next = &LinkedList{2, nil}
+	t4.next.next = &LinkedList{3, nil}
+
+	// define tests output.
+	tests := []struct {
+		in       *LinkedList
+		expected []int
+	}{
+		{t1, []int{0}},
+		{t2, []int{1}},
+		{t3, []int{2, 1}},
+		{t4, []int{3, 2, 1}},
+	}
+
+	for _, tt := range tests {
+		p := reverseLinkedList(tt.in)
+		common.Equal(t, tt.expected, linkedListToSlice(p))
+	}
+}
+
+func reverseLinkedList(node *LinkedList) *LinkedList {
+	current := node
+	var previous, next *LinkedList
+
+	for current != nil {
+		// copy a pointer to the next element.
+		next = current.next
+
+		// reverse the next pointer.
+		current.next = previous
+
+		// step forward in the list.
+		previous = current
+		current = next
+	}
+
+	// return the head of new linked list.
+	return previous
+}