Problems needed to do Twice!

These problem all have some elegant method to finish it.

1. Sudoku Solver (Think about how to construct helper function and ArrayList to mark empty slots)
2. word search (2D boolean array && bugs from these array in recursive function)
3. word ladder
4. Longest Palindromic Substring (Think about 2 ways of constructing palindrome number)
5. Multiply Strings (Think about how to construct an array to store sum of multiplication)