6x6 Sudoku Solver using DFS + Backtracking

• ☆ In puzzles like Sudoku, we often use a strategy called DFS + Backtracking.
• ☆ Here's the typical process:
  • ✅ Make a guess — try placing a number in an empty cell.
  • 🔍 Go deeper — use DFS to continue solving based on that guess.
  • ❌ If it fails — the guess leads to a dead end.
  • ↩️ Backtrack — undo the guess and try a different value.
• ☆ This pattern allows the algorithm to explore all possible combinations while efficiently skipping invalid paths.