hello/reverse: add Int example and its test

hello/reverse/int.go

@@ -0,0 +1,17 @@
+package reverse
+
+import "strconv"
+
+// Int returns its integer argument with its decimal digits reversed,
+// preserving the sign.
+func Int(i int) int {
+	neg := i < 0
+	if neg {
+		i = -i
+	}
+	n, _ := strconv.Atoi(String(strconv.Itoa(i)))
+	if neg {
+		n = -n
+	}
+	return n
+}

hello/reverse/int_test.go

@@ -0,0 +1,26 @@
+package reverse_test
+
+import (
+	"testing"
+
+	"golang.org/x/example/hello/reverse"
+)
+
+func TestInt(t *testing.T) {
+	for _, c := range []struct {
+		in, want int
+	}{
+		{0, 0},
+		{1, 1},
+		{12, 21},
+		{123, 321},
+		{100, 1},
+		{-123, -321},
+		{-1, -1},
+	} {
+		got := reverse.Int(c.in)
+		if got != c.want {
+			t.Errorf("Int(%d) == %d, want %d", c.in, got, c.want)
+		}
+	}
+}