python-algorithms/algorithms/stack/switch_pairs.py at master · ilyvsc/python-algorithms · GitHub

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"""

Given a stack, switch_pairs function takes a stack as a parameter and that

switches successive pairs of numbers starting at the bottom of the stack.

For example, if the stack initially stores these values:

bottom [3, 8, 17, 9, 1, 10] top

Your function should switch the first pair (3, 8),

the second pair (17, 9), ...:

bottom [8, 3, 9, 17, 10, 1] top

if there are an odd number of values in the stack, the value at the top of the

stack is not moved: For example:

bottom [3, 8, 17, 9, 1] top

It would again switch pairs of values, but the value at the

top of the stack (1)

would not be moved

bottom [8, 3, 9, 17, 1] top

Note: There are 2 solutions:

first_switch_pairs: it uses a single stack as auxiliary storage

second_switch_pairs: it uses a single queue as auxiliary storage

"""

import collections

def first_switch_pairs(stack):

storage_stack = []

for i in range(len(stack)):

storage_stack.append(stack.pop())

for i in range(len(storage_stack)):

if len(storage_stack) == 0:

break

first = storage_stack.pop()

if len(storage_stack) == 0: # case: odd number of values in stack

stack.append(first)

break

second = storage_stack.pop()

stack.append(second)

stack.append(first)

return stack

def second_switch_pairs(stack):

q = collections.deque()

# Put all values into queue from stack

for i in range(len(stack)):

q.append(stack.pop())

# Put values back into stack from queue

for i in range(len(q)):

stack.append(q.pop())

# Now, stack is reverse, put all values into queue from stack

for i in range(len(stack)):

q.append(stack.pop())

# Swap pairs by appending the 2nd value before appending 1st value

for i in range(len(q)):

if len(q) == 0:

break

first = q.pop()

if len(q) == 0: # case: odd number of values in stack

stack.append(first)

break

second = q.pop()

stack.append(second)

stack.append(first)

return stack

python-algorithms/algorithms/stack/switch_pairs.py at master · ilyvsc/python-algorithms · GitHub

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