Determine whether an integer is a palindrome. Do this without extra space.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

如果是负数，先转换为正数在处理，处理之后再补上符号位。通过模10、余10可以从低到高的按序获得每一位数字，按照乘10累加即可。注意1：有些数字反转之后会越界，需要特别处理。注意2：负数反转时要特别注意-2^31直接反转时越界的，要特殊处理。

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

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.

There are two sorted arrays nums1 and nums2 of size m and n respectively.

Given a string, find the length of the longest substring without repeating characters.

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Given an array of integers, return indices of the two numbers such that they add up to a specific target.