// IntList.java
//
// CS 101  Class Example
//
// Linked lists of integers.

// To run this code, use "java IntList"

public class IntList {
	
    // Instance variables
	
    private int head;
    private IntList tail;
	
    // Constructor method:
    public IntList(int head, IntList tail) {
	this.head = head;
	this.tail = tail;
    }
	
    public IntList prepend(int n) {
	return new IntList(n, this);
    }
	
    public int head() {
	return head;
    }
	
    public IntList tail() {
	return tail;
    }

    // ----------------------------------------------------------
    // static methods that operate on IntLists
	
    public static int length(IntList L) {
	// return length of L
	if (L == null) {
	    return 0;
	} else {
	    return 1 + length(L.tail());
	}
    }
	
    public static int sum(IntList L) {
	// return sum of all elements of L
	if (L == null) {
	    return 0;
	} else {
	    return L.head() + sum(L.tail());
	}
    }
	
    public static int nth(int n, IntList L) {
	// return n-th number in L (starting with 0)
	if (n == 0) {
	    return L.head();
	} else {
	    return nth(n-1, L.tail());
	}
    }

    // ----------------------------------------------------------
    // toString methods so we can print IntLists
	
    public String toString() {
	// create a string representation of this
	StringBuffer sb = new StringBuffer();
	sb.append("[");
	sb.append(head);
	IntList toDo = tail;
	while (toDo != null) {
	    sb.append(", ");
	    sb.append(toDo.head);
	    toDo = toDo.tail;
	}
	sb.append("]");
	return sb.toString();
    }

    // ----------------------------------------------------------
    // main method for testing:

    public static void main(String[] argv) {
	IntList x = new IntList(3, new IntList(5, null));
	System.out.println("x = " + x);
	x = x.prepend(30);
	System.out.println("x = " + x);
    
	IntList y = new IntList(-6, new IntList(-7, x));
	System.out.println("y = " + y);
	
	System.out.println("length(x) = " + length(x));
	System.out.println("length(y) = " + length(y));
	System.out.println("sum(x) = " + sum(x));
	System.out.println("sum(y) = " + sum(y));
	System.out.println("nth(0, x) = " + nth(0, x));
	System.out.println("nth(1, y) = " + nth(1, y));
	System.out.println("nth(4, y) = " + nth(4, y));
    }
  
}

/*
  Output produced:
  
  x = [3, 5]
  x = [30, 3, 5]
  y = [-6, -7, 30, 3, 5]
  length(x) = 3
  length(y) = 5
  sum(x) = 38
  sum(y) = 25
  nth(0, x) = 30
  nth(1, y) = -7
  nth(4, y) = 5

*/