Learnt Stack from the book.

MayankShah1 · MayankShah1 · commit 415acbc0ebfd · 2017-04-02T12:37:17.000+05:30
diff --git a/.idea/workspace.xml b/.idea/workspace.xml
diff --git a/06-Stacks-Queues-Deques/Stacks/__pycache__/stackarray.cpython-35.pyc b/06-Stacks-Queues-Deques/Stacks/__pycache__/stackarray.cpython-35.pyc
diff --git a/06-Stacks-Queues-Deques/Stacks/matching-delimiters.py b/06-Stacks-Queues-Deques/Stacks/matching-delimiters.py
@@ -0,0 +1,23 @@
+import stackarray as Stack
+
+def is_matched(expr):
+    """Returns True if all the delimiters are matched properly;False otherwise."""
+    S = Stack.ArrayStack()
+    left = '({['
+    right = ')}]'
+    for c in expr:
+        if c in left:
+            S.push(c)
+        elif c in right:
+            if S.is_empty():
+                return False
+            if right.index(c) != left.index(S.pop()):
+                return False
+    return S.is_empty()
+
+# Testing
+if __name__ == "__main__":
+    print(is_matched('[(5+x)-(6-y)]'))
+    print(is_matched('{2*3+(x - 7 + [14-17])}'))
+    print(is_matched('(23(12)]'))
+
diff --git a/06-Stacks-Queues-Deques/Stacks/matching-tags.py b/06-Stacks-Queues-Deques/Stacks/matching-tags.py
@@ -0,0 +1,31 @@
+import stackarray as Stack
+
+def is_matched_html(raw):
+    """Return True if all HTML tags are properly matched;False otherwise"""
+
+    S = Stack.ArrayStack()
+    j = raw.find('<')
+    while j != -1:
+        k = raw.find('>', j+1)
+        if k == -1:                     # invalid tag
+            return False
+        tag = raw[j+1 : k]              # strip away the tag
+        if not tag.startswith('/'):     # opening tag
+            S.push(tag)
+        else:
+            if S.is_empty():
+                return False
+            if tag[1:] != S.pop():
+                return False
+        j = raw.find('<', k+1)
+    return S.is_empty()
+
+# Testing
+if __name__ == "__main__":
+    raw_html = open('/home/mayank/Desktop/Algorithms-in-Python/06-Stacks-Queues-Deques/Stacks/sample.html')
+    html_list = []
+    for line in raw_html:
+        html_list.append(line)
+    html_str = ''.join(html_list)
+    print(html_str)
+    print(is_matched_html(html_str))
diff --git a/06-Stacks-Queues-Deques/Stacks/reverse-file.py b/06-Stacks-Queues-Deques/Stacks/reverse-file.py
@@ -0,0 +1,20 @@
+import stackarray as Stack
+
+def reverse_file(filename):
+    """Overwrite a a given file with its contents line-by-line reversed."""
+    S = Stack.ArrayStack()
+    original = open(filename)
+    for line in original:
+        S.push(line.rstrip('\n'))
+    original.close()
+
+    # Now we overwrite the file
+    output = open(filename,'w')
+    while not S.is_empty():
+        output.write(S.pop() + '\n')
+    output.close()
+
+
+# Testing
+if __name__ == "__main__":
+    reverse_file('/home/mayank/Desktop/Algorithms-in-Python/06-Stacks-Queues-Deques/Stacks/sample.txt')
diff --git a/06-Stacks-Queues-Deques/Stacks/sample.html b/06-Stacks-Queues-Deques/Stacks/sample.html
@@ -0,0 +1,23 @@
+<html>
+<head>
+    <title>Title</title>
+</head>
+<body>
+    <center>
+    <h1> The Little Boat </h1>
+    </center>
+    <p> The storm tossed the little
+    boat like a cheap sneaker in an
+    old washing machine. The three
+    drunken fishermen were used to
+    such treatment, of course, but
+    not the tree salesman, who even as
+    a stowaway now felt that he
+    had overpaid for the voyage. </p>
+    <ol>
+    <li> Will the salesman die? </li>
+    <li> What color is the boat? </li>
+    <li> And what about Naomi? </li>
+    </ol>
+</body>
+</html>
diff --git a/06-Stacks-Queues-Deques/Stacks/sample.txt b/06-Stacks-Queues-Deques/Stacks/sample.txt
@@ -0,0 +1,2 @@
+Let's see whether it works or not.
+Trying out reverse the contents of file using stack.
diff --git a/06-Stacks-Queues-Deques/Stacks/stackarray.py b/06-Stacks-Queues-Deques/Stacks/stackarray.py
@@ -0,0 +1,59 @@
+class Empty(Exception):
+    """Error attempting to access an element from an empty container"""
+    pass
+
+class ArrayStack:
+    """LIFO Stack implementation using a Python list as underlying storage."""
+
+    def __init__(self):
+        """Create an empty stack."""
+        self._data = []
+
+    def __len__(self):
+        """Return the number of elements in the stack."""
+        return len(self._data)
+
+    def is_empty(self):
+        """Return True if stack is empty."""
+        return len(self._data) == 0
+
+    def push(self,e):
+        """Add element e to the top of the stack."""
+        self._data.append(e)
+
+    def top(self):
+        """Return the element at the top of the stack.
+        
+        Raise empty exception if the stack is empty.
+        """
+        if self.is_empty():
+            raise Empty('Stack is empty.')
+        return self._data[-1]
+
+    def pop(self):
+        """Remove and return the element from the top of the Stack.
+        
+        Raise empty exception if the stack is empty.
+        """
+        if self.is_empty():
+            raise Empty('Stack is empty.')
+        return self._data.pop()
+
+
+# Testing
+if __name__ == "__main__":
+    S = ArrayStack()
+    S.push(5)
+    S.push(3)
+    print(len(S))
+    print(S.pop())
+    print(S.is_empty())
+    print(S.pop())
+    print(S.is_empty())
+    S.push(7)
+    S.push(9)
+    print(S.top())
+    S.push(4)
+    print(len(S))
+    print(S.pop())
+    S.push(6)

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Let's see whether it works or not.`
	`2`	`+Trying out reverse the contents of file using stack.`