Add sumsqrdiff solution, unit test and benchmark

sumsqrdiff.go

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+package goexercises
+
+// SumSqrDiff - https://projecteuler.net/problem=6
+// The sum of the squares of the first ten natural numbers is,
+// 12 + 22 + ... + 102 = 385
+// The square of the sum of the first ten natural numbers is,
+// (1 + 2 + ... + 10)2 = 552 = 3025
+// Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is 3025 − 385 = 2640.
+// Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.
+func SumSqrDiff(n int) int {
+	sumOfSq := 0 // Sum of the squares ie. 1^2+2^3...
+	sqSum := 0   // Square of the sum ie.  (1+2+...)^3
+	for i := 1; i <= n; i++ {
+		sumOfSq += i * i
+		sqSum += i
+	}
+	return sqSum*sqSum - sumOfSq
+}

sumsqrdiff_test.go

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+package goexercises
+
+import (
+	"testing"
+)
+
+func TestSumSqrDiff(t *testing.T) {
+	testCases := []struct {
+		input, expected int
+	}{
+		{10, 2640},
+		{100, 25164150},
+	}
+	for _, test := range testCases {
+		observed := SumSqrDiff(test.input)
+		if observed != test.expected {
+			t.Errorf("for input '%d', expected '%d, observed '%d'\n",
+				test.input, test.expected, observed)
+		}
+	}
+}
+
+func BenchmarkSumSqrDiff(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		SumSqrDiff(100)
+	}
+}