Merge pull request #23 from jerryderry/master

implementation of linked-list-based stack in python

Author: wangzheng0822

object-c/08_stack/LinkedStack.h

@@ -0,0 +1,19 @@
+//
+//  LinkedStack.h
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/8.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+//  Stack based upon linked list
+//  基于链表实现的栈
+
+#import <Foundation/Foundation.h>
+
+@interface LinkedStack : NSObject
+
+- (BOOL)isEmpty;
+- (void)push:(int)value;
+- (int)pop;
+
+@end

object-c/08_stack/LinkedStack.m

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+//
+//  LinkedStack.m
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/8.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+#import "LinkedStack.h"
+#import "ListNode.h"
+
+@implementation LinkedStack
+{
+    @private
+    ListNode* _top;
+}
+
+- (BOOL)isEmpty {
+    return _top == nil;
+}
+
+- (void)push:(int)value {
+    ListNode *newTop = [ListNode nodeWithValue:value];
+    newTop.next = _top;
+    _top = newTop;
+}
+
+- (int)pop {
+    if ([self isEmpty]) {
+        [NSException raise:NSRangeException format:@"The stack is empty."];
+    }
+    int value = _top.value;
+    _top = _top.next;
+    return value;
+}
+
+- (NSString *)debugDescription {
+    NSMutableString *info = [[NSMutableString alloc] init];
+    ListNode *current = _top;
+    while (current) {
+        [info appendString:[NSString stringWithFormat:@"%d]", current.value]];
+        current = current.next;
+    }
+    return [NSString stringWithString:info];
+}
+
+@end

object-c/08_stack/LinkedStackTests.m

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+//
+//  LinkedStackTests.m
+//  LinkedStackTests
+//
+//  Created by Wenru Dong on 2018/10/8.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//  
+
+#import <XCTest/XCTest.h>
+#import "LinkedStack.h"
+
+@interface LinkedStackTests : XCTestCase
+
+@end
+
+@implementation LinkedStackTests
+
+- (void)setUp {
+    [super setUp];
+    // Put setup code here. This method is called before the invocation of each test method in the class.
+}
+
+- (void)tearDown {
+    // Put teardown code here. This method is called after the invocation of each test method in the class.
+    [super tearDown];
+}
+
+- (void)testPush {
+    LinkedStack *stack = [[LinkedStack alloc] init];
+    for (int i = 0; i < 10; i++) {
+        [stack push:i];
+    }
+    XCTAssertEqualObjects([stack debugDescription], @"9]8]7]6]5]4]3]2]1]0]");
+}
+
+- (void)testPop {
+    LinkedStack *stack = [[LinkedStack alloc] init];
+    for (int i = 0; i < 10; i++) {
+        [stack push:i];
+    }
+    [stack pop];
+    XCTAssertEqualObjects([stack debugDescription], @"8]7]6]5]4]3]2]1]0]");
+    for (int i = 0; i < 9; i++) {
+        [stack pop];
+    }
+    XCTAssertThrowsSpecificNamed([stack pop], NSException, NSRangeException, @"The stack is empty.");
+}
+
+//- (void)testPerformanceExample {
+//    // This is an example of a performance test case.
+//    [self measureBlock:^{
+//        // Put the code you want to measure the time of here.
+//    }];
+//}
+
+@end

object-c/08_stack/ListNode.h

@@ -0,0 +1,19 @@
+//
+//  ListNode.h
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/6.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+
+@interface ListNode : NSObject
+
+@property int value;
+@property ListNode *next;
+
+- (instancetype)initWithValue:(int)value;
++ (instancetype)nodeWithValue:(int)value;
+
+@end

object-c/08_stack/ListNode.m

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+//
+//  ListNode.m
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/6.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+#import "ListNode.h"
+
+@implementation ListNode
+
+- (instancetype)initWithValue:(int)value {
+    if (self = [super init]) {
+        _value = value;
+    }
+    return self;
+}
+
++ (instancetype)nodeWithValue:(int)value {
+    return [[self alloc] initWithValue:value];
+}
+
+- (NSString*)debugDescription {
+    return [NSString stringWithFormat:@"%d", _value];
+}
+
+@end

python/08_stack/linked_stack.py

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+"""
+    Stack based upon linked list
+    基于链表实现的栈
+    
+    Author: Wenru
+"""
+
+from typing import Optional
+
+class Node:
+    
+    def __init__(self, data: int, next=None):
+        self._data = data
+        self._next = next
+    
+
+class LinkedStack:
+    """A stack based upon singly-linked list.
+    """
+    def __init__(self):
+        self._top: Node = None
+    
+    def push(self, value: int):
+        new_top = Node(value)
+        new_top._next = self._top
+        self._top = new_top
+    
+    def pop(self) -> Optional[int]:
+        if self._top:
+            value = self._top._data
+            self._top = self._top._next
+            return value
+    
+    def __repr__(self) -> str:
+        current = self._top
+        nums = []
+        while current:
+            nums.append(current._data)
+            current = current._next
+        return " ".join(f"{num}]" for num in nums)
+
+if __name__ == "__main__":
+    stack = LinkedStack()
+    for i in range(9):
+        stack.push(i)
+    print(stack)
+    for _ in range(3):
+        stack.pop()
+    print(stack)