Remove nth node from end of list

Author: volodimyr

19.remove_nth_node_from_end_of_list/remove.go

@@ -0,0 +1,52 @@
+// 19. Remove nth node from end of list
+// Topics: 'Two Pointers', 'Linked List'
+// Level: 'Medium'
+
+// Given the head of a linked list, remove the nth node from the end of the list and return its head.
+
+// Example 1:
+
+// Input: head = [1,2,3,4,5], n = 2
+// Output: [1,2,3,5]
+
+// Example 2:
+
+// Input: head = [1], n = 1
+// Output: []
+
+// Example 3:
+
+// Input: head = [1,2], n = 1
+// Output: [1]
+
+// Constraints:
+
+//     The number of nodes in the list is sz.
+//     1 <= sz <= 30
+//     0 <= Node.val <= 100
+//     1 <= n <= sz
+
+package removenthnodefromendoflist
+
+func removeNthFromEnd(head *ListNode, n int) *ListNode {
+	dummy := &ListNode{Next: head}
+
+	l := dummy
+	r := head
+	for n > 0 {
+		r = r.Next
+		n--
+	}
+	for r != nil {
+		r = r.Next
+		l = l.Next
+	}
+	l.Next = l.Next.Next
+
+	return dummy.Next
+}
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}

19.remove_nth_node_from_end_of_list/remove_test.go

@@ -0,0 +1,99 @@
+package removenthnodefromendoflist
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestRemoveNthFromEnd(t *testing.T) {
+	tests := []struct {
+		name string
+		head *ListNode
+		n    int
+		want *ListNode
+	}{
+		{
+			name: "remove 2nd from end in 5-node list",
+			head: createList([]int{1, 2, 3, 4, 5}),
+			n:    2,
+			want: createList([]int{1, 2, 3, 5}),
+		},
+		{
+			name: "remove only node",
+			head: createList([]int{1}),
+			n:    1,
+			want: nil,
+		},
+		{
+			name: "remove last node in 2-node list",
+			head: createList([]int{1, 2}),
+			n:    1,
+			want: createList([]int{1}),
+		},
+		{
+			name: "remove first node in 2-node list",
+			head: createList([]int{1, 2}),
+			n:    2,
+			want: createList([]int{2}),
+		},
+		{
+			name: "remove last node in long list",
+			head: createList([]int{1, 2, 3, 4, 5, 6}),
+			n:    1,
+			want: createList([]int{1, 2, 3, 4, 5}),
+		},
+		{
+			name: "remove first node in long list",
+			head: createList([]int{1, 2, 3, 4, 5}),
+			n:    5,
+			want: createList([]int{2, 3, 4, 5}),
+		},
+		{
+			name: "remove middle node 2",
+			head: createList([]int{1, 2, 3, 4, 5}),
+			n:    4,
+			want: createList([]int{1, 3, 4, 5}),
+		},
+		{
+			name: "remove middle node 3",
+			head: createList([]int{1, 2, 3, 4, 5}),
+			n:    3,
+			want: createList([]int{1, 2, 4, 5}),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := removeNthFromEnd(tt.head, tt.n)
+			if !equalLists(got, tt.want) {
+				t.Errorf("removeNthFromEnd() = %v, want %v", listToSlice(got), listToSlice(tt.want))
+			}
+		})
+	}
+}
+
+func createList(vals []int) *ListNode {
+	if len(vals) == 0 {
+		return nil
+	}
+	head := &ListNode{Val: vals[0]}
+	current := head
+	for i := 1; i < len(vals); i++ {
+		current.Next = &ListNode{Val: vals[i]}
+		current = current.Next
+	}
+	return head
+}
+
+func listToSlice(head *ListNode) []int {
+	var result []int
+	for head != nil {
+		result = append(result, head.Val)
+		head = head.Next
+	}
+	return result
+}
+
+func equalLists(l1, l2 *ListNode) bool {
+	return reflect.DeepEqual(listToSlice(l1), listToSlice(l2))
+}