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

public string LongestPalindrome(string s) {
        if (string.IsNullOrEmpty(s)) return string.Empty;
            List<char> list = s.ToCharArray().ToList();
            int length = list.Count;
            int max = 0;
            List<char> longest = new List<char>();

            if (length == 1)
            {
                longest = new List<char>() {list[0]};
            }

            for (int i = 0; i < length; i++)
            {
                if ((i + 1 < length && list[i] == list[i + 1]))
                {
                    int left, right, sum;
                    List<char> l = new List<char>();

                    left = i;
                    right = i + 1;
                    sum = 1;
                    l.Add(list[i]);
                    l.Add(list[i + 1]);

                    while (left - 1 >= 0 && right + 1 < length)
                    {
                        if (list[left - 1] == list[right + 1])
                        {
                            left = left - 1;
                            right = right + 1;
                            sum++;
                            l.Insert(0, list[left]);
                            l.Add(list[right]);
                        }
                        else
                        {
                            break;
                        }
                    }
                    if (sum >= max)
                    {
                        max = sum;
                        longest = l;
                    }
                }

                if (i > 0 && i + 1 < length && list[i - 1] == list[i + 1])
                {
                    int left, right, sum;
                    List<char> l = new List<char>();
                    left = i - 1;
                    right = i + 1;
                    sum = 2;
                    l.Add(list[i - 1]);
                    l.Add(list[i]);
                    l.Add(list[i + 1]);
                    while (left - 1 >= 0 && right + 1 < length)
                    {
                        if (list[left - 1] == list[right + 1])
                        {
                            left = left - 1;
                            right = right + 1;
                            sum++;
                            l.Insert(0, list[left]);
                            l.Add(list[right]);
                        }
                        else
                        {
                            break;
                        }
                    }
                    if (sum > max)
                    {
                        max = sum;
                        longest = l;
                    }
                }
            }
            return new string(longest.ToArray());
    }

精简版Submission 2 RT:120ms

public string LongestPalindrome(string s)
        {
            if (string.IsNullOrEmpty(s)) return string.Empty;

            int start = 0, end = 0;
            for (int i = 0; i < s.Length; i++)
            {
                int len1 = ExpandAroundCenter(s, i, i);
                int len2 = ExpandAroundCenter(s, i, i+1);
                int len = Math.Max(len1, len2);
                if (len > end - start)
                {
                    start = i - (len - 1)/2;
                    end = i + len/2;
                }
            }
            return s.Substring(start, end - start + 1);
        }

        private int ExpandAroundCenter(string s, int left, int right)
        {
            int l = left, r = right;
            while (l >= 0 && r < s.Length && s[l] == s[r])
            {
                l--;
                r++;
            }
            return r - l - 1;
        }