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Stack.java
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127 lines (116 loc) · 2.77 KB
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package jiuzhang.java.elementary;
public class Stack {
//*Note: good way, it is hard to keep track of previous node, why not operate at the beginning of the list
// beginning of list is the top of the stack, do push as insert element at the head of list
// And also use the dummy to make preInsersion of the head to postInsertion of the dummy
class ListNode {
private int val;
public ListNode next;
ListNode(int val){
this.val = val;
}
}
ListNode dummy;
Stack() {
dummy = new ListNode(-1); //dummy always point to the head of the list
}
public void push(int x) {
// write your code here
ListNode node = new ListNode(x);
node.next = dummy.next;
dummy.next = node;
}
/*
* @return: nothing
*/
public void pop() {
// write your code here
dummy.next = dummy.next.next;
}
/*
* @return: An integer
*/
public int top() {
// write your code here
if (isEmpty()) {
return -1;
}
return dummy.next.val;
}
/*
* @return: True if the stack is empty
*/
public boolean isEmpty() {
// write your code here
return dummy.next == null;
}
//
// //bad way
// /*
// * @param x: An integer
// * @return: nothing
// */
// class ListNode {
// private int val;
// public ListNode next;
// ListNode(int val){
// this.val = val;
// }
// }
//
// private ListNode dummy, tail;
//
// private int size;
//
// Stack() {
// size = 0;
// dummy = new ListNode(0);
// tail = null;
// }
//
//
// public void push(int x) {
// // write your code here
// if (this.size == 0) {
// dummy.next = new ListNode(x);
// tail = dummy.next;
// } else {
// tail.next = new ListNode(x);
// tail = tail.next;
// }
// size++;
// }
//
// /*
// * @return: nothing
// */
// public void pop() {
// // write your code here
// ListNode preTail = dummy;
// while (preTail.next.next != null) { //*Note: this take to much time
// preTail = preTail.next;
// }
// tail = preTail;
// preTail.next = null;
// size--;
// }
//
// /*
// * @return: An integer
// */
// public int top() {
// // write your code here
// if (this.isEmpty()) {
// return -1;
// }
// return tail.val;
// }
//
// /*
// * @return: True if the stack is empty
// */
// public boolean isEmpty() {
// // write your code here
// return size == 0;
// }
}