05.py

'''
	Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

	Example 1:

	Input: "babad"
	Output: "bab"
	Note: "aba" is also a valid answer.
	Example 2:

	Input: "cbbd"
	Output: "bb"
'''

class Solution(object):
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        dp = [[0 for _ in range(len(s))] for _ in range(len(s))]
        maxLength, result = 1, ""
        for index in range(len(s)):
        	dp[index][index] = 1
        	result = s[index]

        length = 2
        
        while length <= len(s):
        	index_i = 0
        	while index_i < len(s) - length + 1:
        		index_j = index_i + length -1

        		if length == 2 and s[index_i] == s[index_j]:
        			dp[index_i][index_j] = 1
        			maxLength = max(maxLength, 2)
        			result = s[index_i:index_j+1]
        		elif s[index_i] == s[index_j] and dp[index_i+1][index_j-1]:
        			dp[index_i][index_j] = 1
        			if length > maxLength:
        				maxLength = length
        				result = s[index_i:index_j+1]

        		index_i += 1
        	length += 1

        return result

# Space: O(N^2)
# Time: O(N^2)

class Solution(object):
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """

        def expand(s, left, right):
        	while left >= 0 and right < len(s) and s[left] == s[right]:
        		left -= 1
        		right += 1
        	return right-left-1

        start, end = 0, 0
        for index in range(len(s)):
        	even_len = expand(s, index, index+1)
        	odd_len = expand(s, index, index)
        	length = max(even_len, odd_len)
        	if length > (end-start):
        		start = index - (length-1)/2
        		end = index +length/2

        return s[start:end+1]