5. Longest Palindromic Substring
Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.
Difficulty: Medium
在字符串中找出回文子字符串，主要需要考虑的问题是
1.回文格式abba,abcba
2.容易出错的情况，aaaa,aaaaa
3.回文计数，abcba>abba
4.采用从中间往两边扩展的方法来计算
Submission 1 RT:568ms
1 public string LongestPalindrome(string s) { 2 if (string.IsNullOrEmpty(s)) return string.Empty; 3 List<char> list = s.ToCharArray().ToList(); 4 int length = list.Count; 5 int max = 0; 6 List<char> longest = new List<char>(); 7 8 if (length == 1) 9 { 10 longest = new List<char>() {list[0]}; 11 } 12 13 for (int i = 0; i < length; i++) 14 { 15 if ((i + 1 < length && list[i] == list[i + 1])) 16 { 17 int left, right, sum; 18 List<char> l = new List<char>(); 19 20 left = i; 21 right = i + 1; 22 sum = 1; 23 l.Add(list[i]); 24 l.Add(list[i + 1]); 25 26 while (left - 1 >= 0 && right + 1 < length) 27 { 28 if (list[left - 1] == list[right + 1]) 29 { 30 left = left - 1; 31 right = right + 1; 32 sum++; 33 l.Insert(0, list[left]); 34 l.Add(list[right]); 35 } 36 else 37 { 38 break; 39 } 40 } 41 if (sum >= max) 42 { 43 max = sum; 44 longest = l; 45 } 46 } 47 48 if (i > 0 && i + 1 < length && list[i - 1] == list[i + 1]) 49 { 50 int left, right, sum; 51 List<char> l = new List<char>(); 52 left = i - 1; 53 right = i + 1; 54 sum = 2; 55 l.Add(list[i - 1]); 56 l.Add(list[i]); 57 l.Add(list[i + 1]); 58 while (left - 1 >= 0 && right + 1 < length) 59 { 60 if (list[left - 1] == list[right + 1]) 61 { 62 left = left - 1; 63 right = right + 1; 64 sum++; 65 l.Insert(0, list[left]); 66 l.Add(list[right]); 67 } 68 else 69 { 70 break; 71 } 72 } 73 if (sum > max) 74 { 75 max = sum; 76 longest = l; 77 } 78 } 79 } 80 return new string(longest.ToArray()); 81 }
精简版Submission 2 RT:120ms
1 public string LongestPalindrome(string s) 2 { 3 if (string.IsNullOrEmpty(s)) return string.Empty; 4 5 int start = 0, end = 0; 6 for (int i = 0; i < s.Length; i++) 7 { 8 int len1 = ExpandAroundCenter(s, i, i); 9 int len2 = ExpandAroundCenter(s, i, i+1); 10 int len = Math.Max(len1, len2); 11 if (len > end - start) 12 { 13 start = i - (len - 1)/2; 14 end = i + len/2; 15 } 16 } 17 return s.Substring(start, end - start + 1); 18 } 19 20 private int ExpandAroundCenter(string s, int left, int right) 21 { 22 int l = left, r = right; 23 while (l >= 0 && r < s.Length && s[l] == s[r]) 24 { 25 l--; 26 r++; 27 } 28 return r - l - 1; 29 }
