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// SORT. Sort a linear singly-linked list of INTs.
class Sort {
// NODE. A node in a linear singly linked list of INTs.
private static class Node {
private int number; // The INT in the node, duh.
private Node next; // The NODE that follows this one, or NULL.
// Constructor. Initialize a new NODE with NUMBER and NEXT.
private Node(int number, Node next) {
this.number = number;
this.next = next;
}
}
// MAKE NODES. Return a list of NODEs that contains INTs from NUMBERS in order
// of their appearance.
private static Node makeNodes(int... numbers) {
if (numbers.length > 0) {
Node first = new Node(numbers[0], null);
Node last = first;
for (int index = 1; index < numbers.length; index += 1) {
last.next = new Node(numbers[index], null);
last = last.next;
}
return first;
} else {
return null;
}
}
// WRITE NODES. Write the INTs from a list of NODEs in paired square brackets,
// separated by commas, with a newline at the end.
private static void writeNodes(Node nodes) {
System.out.print('[');
if (nodes != null) {
System.out.print(nodes.number);
nodes = nodes.next;
while (nodes != null) {
System.out.print(", ");
System.out.print(nodes.number);
nodes = nodes.next;
}
}
System.out.println(']');
}
// SORT NODES. Sort UNSORTED, a list of NODEs, into nondecreasing order of its
// NUMBER slots, without making new NODEs.
private static Node sortNodes(Node unsorted) {
Node left = null, right = null, sorted = null;
if (unsorted==null || unsorted.next==null) {
//unsorted is either empty or of length 1
return unsorted;
} //Halving phase
else {
int step = 1;
while (unsorted != null) {
Node next = unsorted.next;
if (step % 2 == 0) { //Even case
//Pops out first value and puts into Right
unsorted.next = right;
right = unsorted;
} else { //Odd case
// Pops out first value and puts into Left
unsorted.next = left;
left = unsorted;
}
step++;
unsorted = next;
}
}
left = sortNodes(left);
right = sortNodes(right);
return sorted;
}
// MAIN. Run some examples. The comments show what must be printed.
public static void main(String[] args) {
writeNodes(sortNodes(makeNodes())); // []
writeNodes(sortNodes(makeNodes(1))); // [1]
writeNodes(sortNodes(makeNodes(1, 2))); // [1, 2]
writeNodes(sortNodes(makeNodes(2, 1))); // [1, 2]
writeNodes(sortNodes(makeNodes(1, 2, 3, 4, 5, 6, 7, 8, 9)));
// [1, 2, 3, 4, 5, 6, 7, 8, 9]
writeNodes(sortNodes(makeNodes(9, 8, 7, 6, 5, 4, 3, 2, 1)));
// [1, 2, 3, 4, 5, 6, 7, 8, 9]
writeNodes(sortNodes(makeNodes(3, 1, 4, 5, 9, 2, 6, 8, 7)));
// [1, 2, 3, 4, 5, 6, 7, 8, 9]
}
}
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