# Palindrome

#### Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

"A man, a plan, a canal: Panama" -> True
"race a car" -> False

Util methods: isalnum(char), tolower(char)

#### Valid Palindrome Allow One Deletion

Think about Valid Panlindrome allow at most K deletion.

#### Palindrome Pairs [hard]

Given a list of unique words, find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e. words[i] + words[j] is a palindrome.

#### Palindrome Number

Similar problem: reverse integer number

#### Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s.