day 282: add n^2 solution using n space.

day282/problem.go

@@ -1,6 +1,9 @@
 package day282
 
-import "errors"
+import (
+	"errors"
+	"math"
+)
 
 var errNoAnswer = errors.New("no pythagorean triplet exists")
 
@@ -9,7 +12,7 @@ func ErrNoAnswer() error {
 	return errNoAnswer
 }
 
-// PythagoreanTriplet returns the triplets that fulfill
+// PythagoreanTripletBrute returns the triplets that fulfill
 // a^2+b^2=c^2.
 // Runs in O(N^3) time and O(1) space.
 func PythagoreanTripletBrute(nums []int) (a, b, c int, err error) {
@@ -30,3 +33,26 @@ func PythagoreanTripletBrute(nums []int) (a, b, c int, err error) {
 	}
 	return 0, 0, 0, errNoAnswer
 }
+
+// PythagoreanTripletBrute returns the triplets that fulfill
+// a^2+b^2=c^2.
+// Runs in O(N^2) time and O(N) space.
+func PythagoreanTriplet(nums []int) (a, b, c int, err error) {
+	squared := make(map[int]struct{}, len(nums))
+	for _, num := range nums {
+		squared[num*num] = struct{}{}
+	}
+	for i := range nums {
+		for j := range nums {
+			total := nums[i]*nums[i] + nums[j]*nums[j]
+			c := int(math.Sqrt(float64(total)))
+			if _, found := squared[total]; found {
+				if nums[i] < nums[j] {
+					return nums[i], nums[j], c, nil
+				}
+				return nums[j], nums[i], c, nil
+			}
+		}
+	}
+	return 0, 0, 0, errNoAnswer
+}

day282/problem_test.go

@@ -25,6 +25,23 @@ var testcases = []struct {
 	},
 }
 
+func TestPythagoreanTriplet(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if a, b, c, err := PythagoreanTriplet(tc.nums); a != tc.a || b != tc.b || c != tc.c || err != tc.err {
+			t.Errorf("Expected (%v,%v,%v,%v), got (%v,%v,%v,%v)", tc.a, tc.b, tc.c, tc.err, a, b, c, err)
+		}
+	}
+}
+
+func BenchmarkPythagoreanTriplet(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			PythagoreanTriplet(tc.nums) // nolint
+		}
+	}
+}
+
 func TestPythagoreanTripletBrute(t *testing.T) {
 	t.Parallel()
 	for _, tc := range testcases {