Determine whether a Sudoku has a unique solution

Question

I\'m struggling with a backtracking algorithm to determine wether a Sudoku has a unique solution or if it has multiple Solutions. Here\'s the backtracking code i use:

Answer 1

The reset must be inside the for loop and after the if solve condition

 for (int val = 1; val <= 9; ++val) {
        if (legal(i,j,val,cells)) {
            cells[i][j] = val;
            if (solve(i+1,j,cells))
                return true;
            cells[i][j] = 0; // reset on backtrack
        }
    }