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Commit9f3561d

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Yavorskitrekhleb
Yavorski
authored andcommitted
Fix Stack pop comlexity to be O(1) (trekhleb#214)
* By definition Stack push/pop time complexity should be O(1).* Fix is applied by removing head instead of tail in pop method.* Push method now do preprend instead of append.* Fix consistency between toString and toArray methods.
1 parent45fb2a2 commit9f3561d

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2 files changed

+14
-15
lines changed

2 files changed

+14
-15
lines changed

‎src/data-structures/stack/Stack.js

Lines changed: 12 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -4,16 +4,16 @@ export default class Stack {
44
constructor(){
55
// We're going to implement Stack based on LinkedList since these
66
// structures are quite similar. Compare push/pop operations of the Stack
7-
// withappend/deleteTail operations of LinkedList.
7+
// withprepend/deleteHead operations of LinkedList.
88
this.linkedList=newLinkedList();
99
}
1010

1111
/**
1212
*@return {boolean}
1313
*/
1414
isEmpty(){
15-
// The stack is empty if its linked list doesn't have atail.
16-
return!this.linkedList.tail;
15+
// The stack is empty if its linked list doesn't have ahead.
16+
return!this.linkedList.head;
1717
}
1818

1919
/**
@@ -25,27 +25,27 @@ export default class Stack {
2525
returnnull;
2626
}
2727

28-
// Just read the value from theend of linked list without deleting it.
29-
returnthis.linkedList.tail.value;
28+
// Just read the value from thestart of linked list without deleting it.
29+
returnthis.linkedList.head.value;
3030
}
3131

3232
/**
3333
*@param {*} value
3434
*/
3535
push(value){
3636
// Pushing means to lay the value on top of the stack. Therefore let's just add
37-
// the new value at theend of the linked list.
38-
this.linkedList.append(value);
37+
// the new value at thestart of the linked list.
38+
this.linkedList.prepend(value);
3939
}
4040

4141
/**
4242
*@return {*}
4343
*/
4444
pop(){
45-
// Let's try to delete thelast node (thetail) from the linked list.
46-
// If there is notail (the linked list is empty) just return null.
47-
constremovedTail=this.linkedList.deleteTail();
48-
returnremovedTail ?removedTail.value :null;
45+
// Let's try to delete thefirst node (thehead) from the linked list.
46+
// If there is nohead (the linked list is empty) just return null.
47+
constremovedHead=this.linkedList.deleteHead();
48+
returnremovedHead ?removedHead.value :null;
4949
}
5050

5151
/**
@@ -54,8 +54,7 @@ export default class Stack {
5454
toArray(){
5555
returnthis.linkedList
5656
.toArray()
57-
.map(linkedListNode=>linkedListNode.value)
58-
.reverse();
57+
.map(linkedListNode=>linkedListNode.value);
5958
}
6059

6160
/**

‎src/data-structures/stack/__test__/Stack.test.js

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -13,7 +13,7 @@ describe('Stack', () => {
1313
stack.push(1);
1414
stack.push(2);
1515

16-
expect(stack.toString()).toBe('1,2');
16+
expect(stack.toString()).toBe('2,1');
1717
});
1818

1919
it('should peek data from stack',()=>{
@@ -58,7 +58,7 @@ describe('Stack', () => {
5858

5959
conststringifier=value=>`${value.key}:${value.value}`;
6060

61-
expect(stack.toString(stringifier)).toBe('key1:test1,key2:test2');
61+
expect(stack.toString(stringifier)).toBe('key2:test2,key1:test1');
6262
expect(stack.pop().value).toBe('test2');
6363
expect(stack.pop().value).toBe('test1');
6464
});

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