Add: Longest Palindromic Substring

problems/longest-palindromic-substring/longest_palindromic_substring.go

@@ -1 +1,20 @@
 package longest_palindromic_substring
+
+func longestPalindrome(s string) string {
+	start, maxLen, l := 0, 0, len(s)
+	for i := 0; i < l; i++ {
+		expandAroundCenter(s, i, i, &start, &maxLen)
+		expandAroundCenter(s, i, i+1, &start, &maxLen)
+	}
+	return s[start : start+maxLen]
+}
+
+func expandAroundCenter(s string, l, r int, start, maxLen *int) {
+	for l >= 0 && r < len(s) && s[l] == s[r] {
+		l, r = l-1, r+1
+	}
+	if r-l-1 > *maxLen {
+		*maxLen = r - l - 1
+		*start = l + 1
+	}
+}

problems/longest-palindromic-substring/longest_palindromic_substring_test.go

@@ -1 +1,39 @@
 package longest_palindromic_substring
+
+import "testing"
+
+type caseType struct {
+	input    string
+	expected string
+}
+
+func TestLongestPalindrome(t *testing.T) {
+	tests := [...]caseType{
+		{
+			input:    "babad",
+			expected: "bab",
+		},
+		{
+			input:    "cbbd",
+			expected: "bb",
+		},
+		{
+			input:    "",
+			expected: "",
+		},
+		{
+			input:    "a",
+			expected: "a",
+		},
+		{
+			input:    "abbba",
+			expected: "abbba",
+		},
+	}
+	for _, tc := range tests {
+		output := longestPalindrome(tc.input)
+		if output != tc.expected {
+			t.Fatalf("input: %v, output: %v, expected: %v", tc.input, output, tc.expected)
+		}
+	}
+}