Sudoku Solver - Constraint Propagation

Use three sets to track constraints: $1.$ rows[i]: digits used in row $i$.

$2.$ cols[j]: digits used in column $j$.

$3.$ boxes[i/3][j/3]: digits used in that $3 \times 3$ box.

For each empty cell, try digits $1$-$9$. If a digit satisfies all constraints, place it and recurse. Backtrack if no valid digit exists.