Add stack questions

Author: prabhupant

data_structures/stack/remove_duplicates_adjacent.py

@@ -0,0 +1,8 @@
+def remove(s):
+    stack = []
+    for element in s:
+        if stack and element == stack[-1]:
+            stack.pop()
+        else:
+            stack.append(element)
+    

data_structures/stack/valid_parenthesis.py

@@ -0,0 +1,12 @@
+def check(s):
+    stack = []
+    for element in s:
+        if element == '(':
+            stack.append(')')
+        elif element == '[':
+            stack.append(']')
+        elif element == '{':
+            stack.append('}')
+        elif not stack or stack.pop() != element:
+            return False
+    return not stack

data_structures/stack/validate_stack_sequence.py

@@ -0,0 +1,14 @@
+# pushed = [1,2,3,4,5]
+# popped = [4,5,3,2,1]
+
+# this sequence is possible
+
+def verify(pushed, popped):
+    i = 0
+    stack = []
+    for x in pushed:
+        stack.append(x)
+        while stack and i < len(popped) and stack[-1] == popped[i]:
+            stack.pop()
+            i += 1
+    return i == len(popped)