Given a board and a word, find if the word exists in the board.
Sample Input:
[["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], "ABCCED"
Hints
Hint 1
Use DFS with backtracking
Hint 2
Mark visited cells to avoid reuse
Hint 3
Check all four directions from each cell
