Merge 58fd72e into 4b9513f

README.md

@@ -33,3 +33,4 @@ problems from
 * [Day 19 N-Queens](https://github.com/vaskoz/dailycodingproblem-go/issues/43)
 * [Day 19 Flight Itinerary](https://github.com/vaskoz/dailycodingproblem-go/issues/45)
 * [Day 19 Sudoku Solver](https://github.com/vaskoz/dailycodingproblem-go/issues/47)
+* [Day 20](https://github.com/vaskoz/dailycodingproblem-go/issues/49)

day20/problem.go

@@ -0,0 +1,40 @@
+package day20
+
+// Node is a singly linked list. NOTE: Doubly-linked list won't work here with shared paths.
+type Node struct {
+	Value interface{}
+	Next  *Node
+}
+
+// FindCommonNode finds the common element in a linked list in O(N+M) time and O(1) space.
+func FindCommonNode(one, two *Node) *Node {
+	var oneLen, twoLen int
+	for e := one; e != nil; e = e.Next {
+		oneLen++
+	}
+	for e := two; e != nil; e = e.Next {
+		twoLen++
+	}
+	var longer, shorter *Node
+	var diff int
+	if oneLen > twoLen {
+		longer = one
+		shorter = two
+		diff = oneLen - twoLen
+	} else {
+		longer = two
+		shorter = one
+		diff = twoLen - oneLen
+	}
+	for i := 0; i < diff; i++ {
+		longer = longer.Next
+	}
+	for longer != nil {
+		if longer == shorter {
+			return longer
+		}
+		longer = longer.Next
+		shorter = shorter.Next
+	}
+	return nil
+}

day20/problem_test.go

@@ -0,0 +1,48 @@
+package day20
+
+import (
+	"testing"
+)
+
+func createTwoSharedLists() (*Node, *Node, *Node) {
+	common := &Node{1015, &Node{444, nil}}
+	one := &Node{"abc", &Node{"def", &Node{"ghi", common}}}
+	two := &Node{111, common}
+	return one, two, common
+}
+
+func createTwoIndependentLists() (*Node, *Node, *Node) {
+	one := &Node{"abc", &Node{"def", &Node{"ghi", nil}}}
+	two := &Node{111, nil}
+	return one, two, nil
+}
+
+func TestFindCommonNode(t *testing.T) {
+	t.Parallel()
+	one, two, common := createTwoSharedLists()
+	if result := FindCommonNode(one, two); result != common {
+		t.Error("Expected these two be the same pointer")
+	}
+	one, two = two, one // swap
+	if result := FindCommonNode(one, two); result != common {
+		t.Error("Expected these two be the same pointer")
+	}
+	one, two, common = createTwoIndependentLists()
+	if result := FindCommonNode(one, two); result != common {
+		t.Error("Expected these two be nil")
+	}
+	one, two = two, one // swap
+	if result := FindCommonNode(one, two); result != common {
+		t.Error("Expected these two be nil")
+	}
+}
+
+func BenchmarkFindCommonNode(b *testing.B) {
+	one, two, _ := createTwoSharedLists()
+	onei, twoi, _ := createTwoIndependentLists()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		FindCommonNode(one, two)
+		FindCommonNode(onei, twoi)
+	}
+}

sudoku/problem_test.go

@@ -32,6 +32,7 @@ var testcases = []struct {
 }
 
 func TestSolver(t *testing.T) {
+	t.Parallel()
 	for _, tc := range testcases {
 		if result, _ := Solver(tc.board, tc.n); !reflect.DeepEqual(result, tc.expected) {
 			t.Errorf("Expected %v but got %v", tc.expected, result)