001
002
003
004
005
006
007
008
009
010
011
012
013
014
015
016
017
018
019
020
021
022
023
024
025
026
027
028
029
030
031
032
033
034
035
036
037
038
039
040
041
042
043
044
045
046
047
048
049
050
051
052
053
054
055
056
057
058
059
060
061
062
063
064
065
066
067
068
069
070
071
072
073
074
075
076
077
078
079
080
081
082
083
084
085
086
087
088
089
090
091
092
093
094
095
096
097
098
099
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
package algs13;
import  stdlib.*;
import java.util.Iterator;
import java.util.NoSuchElementException;
/* ***********************************************************************
 *  Compilation:  javac Stack.java
 *  Execution:    java Stack < input.txt
 *
 *  A generic stack, implemented using a linked list. Each stack
 *  element is of type Item.
 *
 *  % more tobe.txt
 *  to be or not to - be - - that - - - is
 *
 *  % java Stack < tobe.txt
 *  to be not that or be (2 left on stack)
 *
 *************************************************************************/

/**
 *  The <tt>Stack</tt> class represents a last-in-first-out (LIFO) stack of generic items.
 *  It supports the usual <em>push</em> and <em>pop</em> operations, along with methods
 *  for peeking at the top item, testing if the stack is empty, and iterating through
 *  the items in LIFO order.
 *  <p>
 *  All stack operations except iteration are constant time.
 *  <p>
 *  For additional documentation, see <a href="/algs4/13stacks">Section 1.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 */
public class Stack<T> implements Iterable<T> {
  private int N;          // size of the stack
  private Node<T> first;     // top of stack

  // helper linked list class
  private static class Node<T> {
    public Node () { }
    public T item;
    public Node<T> next;
  }

  /**
   * Create an empty stack.
   */
  public Stack() {
    this.first = null;
    this.N = 0;
  }

  /**
   * Is the stack empty?
   */
  public boolean isEmpty() {
    return first == null;
  }

  /**
   * Return the number of items in the stack.
   */
  public int size() {
    return N;
  }

  /**
   * Add the item to the stack.
   */
  public void push(T item) {
    Node<T> oldfirst = first;
    first = new Node<>();
    first.item = item;
    first.next = oldfirst;
    N++;
  }

  /**
   * Delete and return the item most recently added to the stack.
   * @throws java.util.NoSuchElementException if stack is empty.
   */
  public T pop() {
    if (isEmpty()) throw new NoSuchElementException("Stack underflow");
    T item = first.item;        // save item to return
    first = first.next;            // delete first node
    N--;
    return item;                   // return the saved item
  }


  /**
   * Return the item most recently added to the stack.
   * @throws java.util.NoSuchElementException if stack is empty.
   */
  public T peek() {
    if (isEmpty()) throw new NoSuchElementException("Stack underflow");
    return first.item;
  }

  /**
   * Return string representation.
   */
  public String toString() {
    StringBuilder s = new StringBuilder();
    for (T item : this)
      s.append(item + " ");
    return s.toString();
  }


  // check internal invariants
  private static <T> boolean check(Stack<T> that) {
    int N = that.N;
    Stack.Node<T> first = that.first;
    if (N == 0) {
      if (first != null) return false;
    }
    else if (N == 1) {
      if (first == null)      return false;
      if (first.next != null) return false;
    }
    else {
      if (first.next == null) return false;
    }

    // check internal consistency of instance variable N
    int numberOfNodes = 0;
    for (Stack.Node<T> x = first; x != null; x = x.next) {
      numberOfNodes++;
    }
    if (numberOfNodes != N) return false;

    return true;
  }


  /**
   * Return an iterator to the stack that iterates through the items in LIFO order.
   */
  public Iterator<T> iterator()  { return new ListIterator();  }

  // an iterator, doesn't implement remove() since it's optional
  private class ListIterator implements Iterator<T> {
    private Node<T> current = first;
    public void remove() { throw new UnsupportedOperationException(); }
    public boolean hasNext() { return current != null; }
    //public ListIterator () { TraceGraph.draw (); }
    public T next() {
      if (!hasNext()) throw new NoSuchElementException();
      T item = current.item;
      current = current.next;
      return item;
    }
  }

  /**
   * A test client.
   */
  public static void bookMain(String[] args) {
    StdIn.fromString ("to be or not to - be - - that - - - is");
    Stack<String> stack = new Stack<>();
    while (!StdIn.isEmpty()) {
      String item = StdIn.readString();
      if (!item.equals("-")) stack.push(item);
      else if (!stack.isEmpty()) StdOut.print(stack.pop() + " ");
    }
    StdOut.println(stack.size() + " left on stack:");
    for (String s : stack) {
      StdOut.print (s + " ");
    }
    StdOut.println ();
  }
  public static void main(String[] args) {
    //Trace.showBuiltInObjectsVerbose (true);
    Trace.drawStepsOfMethod ("main");
    Trace.run ();
    Integer r1 = null;
    Stack<Integer> s1 = new Stack<>();
    s1.push (11);
    s1.push (21);
    s1.push (31);
    s1.push (41);
    s1.push (51);
    r1 = s1.pop ();
    r1 = s1.pop ();
    r1 = s1.pop ();
    r1 = null;
    s1.push (61);
    s1.push (71);
    
    String r2 = null;
    Stack<String> s2 = new Stack<>();   
    s2.push ("a");
    s2.push ("b");
    s2.push ("c");
    s2.push ("d");
    s2.push ("e");
    r2 = s2.pop ();
    r2 = s2.pop ();
    r2 = s2.pop ();
    r2 = null;
    s2.push ("f");
    s2.push ("g");
    s2.push ("h");
  }
  public static void main2(String[] args) {
    Trace.showObjectIdsRedundantly (true);
    //Trace.showBuiltInObjectsVerbose (true);
    Trace.drawStepsOfMethod ("main");
    Trace.run ();
    Stack<Integer> s1 = new Stack<>();
    s1.push (300);
    Stack<String> s2 = new Stack<>();
    s2.push ("duck");
    s2.push ("goose");
  }
}