Leetcode 247. Strobogrammatic Number II

Generate all length-n strobogrammatic numbers—strings that appear identical when rotated 180°—by constructing valid digit pairs (0-0,1-1,6-9,8-8,9-6). The core challenge is using recursion/backtracking to place pairs from the outside in while avoiding leading zeros for n>1 and handling the single middle digit when n is odd.