@param a, b: Two integer return: An integer

@param n the nth @return the nth sequence

@param n an integer @return true if this is a happy number or false

@param a, b, the root of binary trees. @return true if they are identical, or false.

@param s A string @return whether the string is a valid parentheses

@param s A string @return the length of last word

@param nums: A list of integers. @return: An integer denotes the middle number of the array.

@param n a positive integer @return an integer

@param digits a number represented as an array of digits @return the result

@param s the IP string @return All possible valid IP addresses

@param matrix a matrix of m x n elements @return an integer list