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5. Longest Palindromic Substring
Medium
Given a string s, return the longest palindromic substring in s.
Example 1:
Input: s = “babad”
Output: “bab” Note: “aba” is also a valid answer.
Example 2:
Input: s = “cbbd”
Output: “bb”
Example 3:
Input: s = “a”
Output: “a”
Example 4:
Input: s = “ac”
Output: “a”
Constraints:
1 <= s.length <= 1000sconsist of only digits and English letters.
Solution
# @param {String} s
# @return {String}
def longest_palindrome(s)
new_str = Array.new(s.length * 2 + 1, 0)
new_str[0] = '#'
(0...s.length).each do |i|
new_str[2 * i + 1] = s[i]
new_str[2 * i + 2] = '#'
end
dp = Array.new(new_str.length, 0)
friend_center = 0
friend_radius = 0
lps_center = 0
lps_radius = 0
(0...new_str.length).each do |i|
dp[i] = friend_center + friend_radius > i ? [dp[2 * friend_center - i], friend_center + friend_radius - i].min : 1
while i + dp[i] < new_str.length && i - dp[i] >= 0 && new_str[i + dp[i]] == new_str[i - dp[i]]
dp[i] += 1
end
if friend_center + friend_radius < i + dp[i]
friend_center = i
friend_radius = dp[i]
end
lps_center, lps_radius = i, dp[i] if lps_radius < dp[i]
end
s[(lps_center - lps_radius + 1) / 2, lps_radius - 1]
end