Palindrome

A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward as forward, such as madam or racecar.

Here are ways to determine if a string is a palindrome:

Reverse the string and it should be equal to itself.
Have two pointers at the start and end of the string. Move the pointers inward till they meet. At every point in time, the characters at both pointers should match.

The order of characters within the string matters, so hash tables are usually not helpful.

When a question is about counting the number of palindromes, a common trick is to have two pointers that move outward, away from the middle.

Note that palindromes can be even or odd length. For each middle pivot position, you need to check it twice - once that includes the character and once without the character.
For substrings, you can terminate early once there is no match