5. Longest Palindromic Substring

leetcode/0005.Longest-Palindromic-Substring/5. Longest Palindromic Substring.go

@@ -0,0 +1,46 @@
+package leetcode
+
+func LongestPalindromicSubstring(s string) string {
+	longestLen := 0
+	longestString := ""
+	runeSlice := []rune(s)
+	for index := range runeSlice {
+		// odd mode
+		length := 1
+		leftIndex := index - 1
+		rightIndex := index + 1
+		for leftIndex >= 0 && rightIndex < len(s) {
+			if runeSlice[leftIndex] == runeSlice[rightIndex] {
+				length += 2
+				leftIndex--
+				rightIndex++
+			} else {
+				break
+			}
+		}
+		if length > longestLen {
+			longestLen = length
+			longestString = string(runeSlice[leftIndex+1 : rightIndex])
+		}
+		// even mode
+		length = 0
+		leftIndex = index
+		rightIndex = index + 1
+		for leftIndex >= 0 && rightIndex < len(s) {
+			if runeSlice[leftIndex] == runeSlice[rightIndex] {
+				length += 2
+				leftIndex--
+				rightIndex++
+			} else {
+				break
+			}
+		}
+
+		if length > longestLen {
+			longestLen = length
+			longestString = string(runeSlice[leftIndex+1 : rightIndex])
+		}
+	}
+
+	return longestString
+}

leetcode/0005.Longest-Palindromic-Substring/5. Longest Palindromic Substring_test.go

@@ -0,0 +1,45 @@
+package leetcode
+
+import (
+	"reflect"
+	"testing"
+)
+
+// 填入 function input type
+type parameters0005 struct {
+	s string
+}
+
+func TestLongestPalindromicSubstring(t *testing.T) {
+	tests := []struct {
+		parameters0005
+		// 填入 function output type
+		ans string
+	}{
+		// 填入 test case
+		{
+			parameters0005{"babad"},
+			"bab",
+		},
+		{
+			parameters0005{"cbbd"},
+			"bb",
+		},
+		{
+			parameters0005{"abcd"},
+			"a",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run("Test LongestPalindromicSubstring", func(t *testing.T) {
+			// 完整輸入參數
+			result := LongestPalindromicSubstring(test.parameters0005.s)
+			// compare 的方式需視情況調整
+			// 由於答案可能不只一組，所以用長度來辨別
+			if !reflect.DeepEqual(len(result), len(test.ans)) {
+				t.Errorf("LongestPalindromicSubstring(%+v) got %+v, want %+v", test.parameters0005, result, test.ans)
+			}
+		})
+	}
+}

leetcode/0005.Longest-Palindromic-Substring/README.md

@@ -0,0 +1,20 @@
+# 5. Longest Palindromic Substring
+
+<https://leetcode.com/problems/longest-palindromic-substring/>
+
+要找出字串 `s` 中的最長回文  
+回文的定義是從頭和從尾讀的順序都一樣的字串  
+（單一個字元也算是回文）
+
+`s` 中可能有不只一組的最長字串，只需要找到其中一組即可
+
+回文根據有或沒有中心點而有兩種形式  
+例如：`abc|cba`, `bbbcc|ccbbb` 屬於左右對稱型  
+例如：`abc|e|cba`, `bbbcc|d|ccbbb` 屬於有中心點的類型  
+
+要找特定字元的最長迴文，時間複雜度是 $O(n)$  
+找完所有字元的迴文，時間複雜度是 $O(n^2)$
+
+## Takeaway
+
+