day 202: faster version without extra space

day202/problem.go

@@ -16,3 +16,23 @@ func IsIntegerPalindrome(i int) bool {
 	}
 	return true
 }
+
+// IsIntegerPalindromeFaster returns true if the integer is a
+// palindrome.
+// Runs in O(digits) time and O(1) space.
+func IsIntegerPalindromeFaster(i int) bool {
+	divisor := 1
+	for temp := i; temp/divisor > 10; divisor = divisor * 10 {
+	}
+	for i >= 10 {
+		highest := i / divisor
+		lowest := i % 10
+		if highest != lowest {
+			return false
+		}
+		i %= divisor
+		i /= 10
+		divisor /= 100
+	}
+	return true
+}

day202/problem_test.go

@@ -9,6 +9,9 @@ var testcases = []struct {
 	{888, true},
 	{121, true},
 	{678, false},
+	{123124234323, false},
+	{123454321, true},
+	{23455432, true},
 }
 
 func TestIsIntegerPalindrome(t *testing.T) {
@@ -27,3 +30,20 @@ func BenchmarkIsIntegerPalindrome(b *testing.B) {
 		}
 	}
 }
+
+func TestIsIntegerPalindromeFaster(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result := IsIntegerPalindromeFaster(tc.num); result != tc.expected {
+			t.Errorf("For input %v, expected %v, got %v", tc.num, tc.expected, result)
+		}
+	}
+}
+
+func BenchmarkIsIntegerPalindromeFaster(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			IsIntegerPalindromeFaster(tc.num)
+		}
+	}
+}