You have n  tiles, where each tile has one letter tiles[i] printed on it.
Return the number of possible non-empty sequences of letters you can make using the letters printed on those tiles.
Example 1:
Input: tiles = "AAB"
Output: 8
Explanation: The possible sequences are "A", "B", "AA", "AB", "BA", "AAB", "ABA", "BAA".
Example 2:
Input: tiles = "AAABBC"
Output: 188
Example 3:
Input: tiles = "V"
Output: 1
Constraints:
1 <= tiles.length <= 7
tiles consists of uppercase English letters.
class Solution:
    def numTilePossibilities(self, tiles: str) -> int:
        def backtracking(idx=0,seq='',remaining=tiles):
            for i, tile in enumerate(remaining):
                res.add(seq+tile)
                backtracking(idx+1, seq+tile, remaining[:i]+remaining[i+1:])
                
        res = set()
        backtracking()
        return len(res)
class Solution:
    def numTilePossibilities(self, tiles: str) -> int:
        def backtracking(idx=0,seq='',remaining=tiles):
            for i, tile in enumerate(remaining):
                res.add(seq+tile)
                backtracking(idx+1, seq+tile, remaining[:i]+remaining[i+1:])
                
        res = set()
        backtracking()
        return len(res)