def is_palindrome(node):
if node is None or node.nxt is None:
return True
s = Stack()
a = b = node
s.push(b)
l = 1
while a is not None and a.nxt is not None:
a = a.nxt
l += 1
b = b.nxt
s.push(b)
a = a.nxt
if a:
l += 1
s.pop()
if l % 2 != 0:
b = b.nxt
while b is not None:
if b.data != s.pop().data:
return False
b = b.nxt
return True
def find_celebrity(array):
s = Stack()
for ppl in array:
s.push(ppl)
a = s.pop()
b = s.pop()
while not s.is_empty():
if does_a_know_b(a, b):
a = s.pop()
else:
b = s.pop()
if does_a_know_b(a, b):
c = b
else:
c = a
for person in array:
if does_a_know_b(c, person) or not does_a_know_b(person, c):
return None
return c
def evaluate(exp):
s = Stack()
i = 0
while i < len(exp):
if not is_operator(exp[i]):
s.push(int(exp[i]))
else:
val1 = s.pop()
val2 = s.pop()
if exp[i] == '+':
s.push(val1 + val2)
elif exp[i] == '-':
s.push(val2 - val1)
elif exp[i] == '*':
s.push(val1 * val2)
elif exp[i] == '/':
s.push(val2 / val1)
else:
s.push(val2 % val1)
return s.pop()
def is_leaf_order_same(root1, root2):
s1 = Stack()
s2 = Stack()
done1 = False
done2 = False
val1 = None
val2 = None
curr1 = root1
curr2 = root2
while True:
while not done1:
if curr1:
s1.push(curr1)
curr1 = curr1.left
else:
if s1.is_empty():
done1 = True
val1 = None
else:
curr1 = s1.pop()
val1 = curr1.data
if is_leaf(curr1):
done1 = True
curr1 = curr1.right
while not done2:
if curr2:
s2.push(curr2)
curr2 = curr2.left
else:
if s2.is_empty():
done2 = True
val2 = None
else:
curr2 = s2.pop()
val2 = curr2.data
if is_leaf(curr2):
done2 = True
curr2 = curr2.right
if done1 and done2:
if val1 != val2:
return False
else:
done1 = False
done2 = False
if val1 is None and val2 is None:
return True
if val1 is None or val2 is None:
return False
def is_sum_pair_present(root, target):
s1 = Stack()
s2 = Stack()
done1 = False
done2 = False
val1 = None
val2 = None
curr1 = root
curr2 = root
while True:
# in order
while not done1:
if curr1:
s1.push(curr1)
curr1 = curr1.left
else:
if s1.is_empty():
done1 = True
else:
curr1 = s1.pop()
val1 = curr1.data
curr1 = curr1.right
done1 = True
# reverse in order
while not done2:
if curr2:
s2.push(curr2)
curr2 = curr2.right
else:
if s2.is_empty():
done2 = True
else:
curr2 = s2.pop()
val2 = curr2.data
curr2 = curr2.left
done2 = True
# val1 != val2 makes sure that two of same values are not added
if val1 != val2 and val1 + val2 == target:
print 'pair found: {0} and {1}'.format(val1, val2)
return True
elif val1 + val2 < target:
done1 = False
elif val1 + val2 > target:
done2 = False
if val2 <= val1:
return False
def is_valid(string):
s = Stack()
for c in string:
if c == 'a':
s.push(c)
elif c == 'b' and not s.is_empty():
s.pop()
else:
return False
return s.is_empty()
def nge(array):
s = Stack()
for num in array:
while not s.is_empty() and s.peek() < num:
print s.pop(), '-', num
s.push(num)
while not s.is_empty():
print s.pop(), '-', -1
def simplify_linux_dir_path(path):
places = [p for p in path.split("/") if p != "." and p != ""]
s = Stack()
for p in places:
if p == ".." and not s.is_empty():
s.pop()
else:
s.push(p)
return "/" + '/'.join(s.get_list())
def build_lowest_number_removing_k_digits(num, k):
s = Stack()
l = len(num) - k
for c in num:
if not s.is_empty() and int(s.peek()) > int(c) and k > 0:
s.pop()
k -= 1
s.push(c)
return ''.join(s.get_list()[:l])
def nge(array):
_nge = [-1 for i in array]
s = Stack()
for i in range(len(array)):
element = array[i]
while not s.is_empty() and element > array[s.peek()]:
_nge[s.peek()] = i
s.pop()
s.push(i)
return _nge
def get_balanced_stack(string):
s = Stack()
for c in string:
if c == '{' or s.is_empty():
s.push(c)
else:
if s.peek() == '{':
s.pop()
else:
s.push(c)
return s
def dupe_found(string):
s = Stack()
for c in string:
exp = ""
if c != ')':
s.push(c)
else:
while not s.is_empty() and s.peek() != '(':
char = s.pop()
exp += char
if not is_exp(exp):
return True
s.pop()
return False
def largest_valid_substring(string):
stack = Stack()
l_index = None
r_index = None
min_index = None
max_index = None
max_diff = 0
for i in range(len(string)):
if string[i] == '(':
stack.push(i)
elif string[i] == ')':
try:
l_index = stack.pop()
r_index = i
except Exception:
pass
if None not in [l_index, r_index]:
diff = r_index - l_index
if diff > max_diff:
max_diff = diff
min_index = l_index
max_index = r_index
if None in [l_index, r_index]:
return None
else:
return string[min_index:max_index + 1]
def compute_left_smaller_diffs(array):
s = Stack()
diff_array = [None for i in array]
for i in range(len(array)):
element = array[i]
while not s.is_empty() and s.peek() > element:
s.pop()
if s.is_empty():
diff_array[i] = 0
else:
diff_array[i] = s.peek()
s.push(element)
return diff_array
def calculate_span(stocks):
spans = [-1 for i in stocks]
s = Stack()
s.push(-1)
spans[0] = 1
for i in range(len(stocks)):
# pop until we find a taller element in stack
while s.peek() != -1 and stocks[s.peek()] <= stocks[i]:
s.pop()
# calculate span
spans[i] = i - s.peek()
s.push(i)
return spans
def iterative_post_order(root):
s1 = Stack()
s2 = Stack()
s1.push(root)
while not s1.is_empty():
node = s1.pop()
if node.left:
s1.push(node.left)
if node.right:
s1.push(node.right)
s2.push(node)
while not s2.is_empty():
print s2.pop(),
def find_largest_area(hist):
s = Stack()
n = len(hist)
max_area = 0
i = 0
while i < n:
# Push as long as stack top is smaller
if s.is_empty() or hist[s.peek()] <= hist[i]:
s.push(i)
i += 1
else:
# pop one by one and calculate area considering, the popped
# elements height as the smallest. notice we are not
# incrementing i here. Hence we keep popping until we find
# a smaller height
s_top = s.pop()
# width is from top to i, but not including i
width = i if s.is_empty() else i - s.peek() - 1
area_with_top_as_smallest = hist[s_top] * width
if max_area < area_with_top_as_smallest:
max_area = area_with_top_as_smallest
# pop entire stack to consider all elements
while not s.is_empty():
s_top = s.pop()
# here i is N, so width will be from top to N, not
# including N
width = i if s.is_empty() else i - s.peek() - 1
area_with_top_as_smallest = hist[s_top] * width
if max_area < area_with_top_as_smallest:
max_area = area_with_top_as_smallest
return max_area
class Queue2(object):
def __init__(self):
self.stack1 = Stack()
self.stack2 = Stack()
def push(self, element):
self.stack1.push(element)
def pop(self):
if self.stack2.is_empty():
while not self.stack1.is_empty():
self.stack2.push(self.stack1.pop())
return self.stack2.pop()
def peek(self):
if self.stack2.is_empty():
while not self.stack1.is_empty():
self.stack2.push(self.stack1.pop())
return self.stack2.peek()
class Queue3(object):
def __init__(self):
self.stack = Stack()
self.element_to_pop = None
self.element_to_peek = None
def push(self, element):
self.stack.push(element)
def _pop(self):
if not self.stack.is_empty():
el = self.stack.pop()
self._pop()
if self.element_to_pop is None:
self.element_to_pop = el
else:
self.stack.push(el)
def _peek(self):
if not self.stack.is_empty():
el = self.stack.pop()
self._pop()
if self.element_to_pop is None:
self.element_to_pop = el
self.stack.push(el)
def pop(self):
self._pop()
el = self.element_to_pop
self.element_to_pop = None
return el
def peek(self):
self._pop()
el = self.element_to_pop
self.element_to_peek = None
return el
def convert(exp):
k = -1
exp = list(exp)
s = Stack()
for i in range(len(exp)):
if is_operand(exp[i]):
k += 1
exp[k] = exp[i]
elif exp[i] == '(':
s.push(exp[i])
elif exp[i] == ')':
while not s.is_empty() and s.peek() != '(':
k += 1
exp[k] = s.pop()
if not s.is_empty() and s.peek() != '(':
return -1
else:
s.pop() # pop off the '('
else:
while not s.is_empty() and precedence(exp[i]) <= precedence(
s.peek()):
k += 1
exp[k] = s.pop()
s.push(exp[i])
while not s.is_empty():
k += 1
exp[k] = s.pop()
return ''.join(exp)[0:k + 1]
def iterative_pre_order(root):
s = Stack()
s.push(root)
while not s.is_empty():
node = s.pop()
print node,
if node.right:
s.push(node.right)
if node.left:
s.push(node.left)
def find_max_len(string):
n = len(string)
result = 0
s = Stack()
s.push(-1)
for i in range(n):
if string[i] == '(':
s.push(i)
else:
s.pop()
if not s.is_empty():
result = max(result, i - s.peek())
else:
s.push(i)
return result
def iterative_dfs(graph, source):
s = Stack()
s.push(source)
visited = [False for i in graph.keys()]
while not s.is_empty():
node = s.pop()
visited[node] = True
for neighbour in graph[node]:
if not visited[neighbour]:
s.push(neighbour)
def print_min_number(sequence):
result = []
s = Stack()
for i in range(len(sequence) + 1):
s.push(i + 1)
if i == len(sequence) or sequence[i] == 'I':
while not s.is_empty():
result.append(s.pop())
print ''.join(map(str, result))
def is_pre_order(array):
s = Stack()
root = -maxint
for value in array:
while not s.is_empty() and s.peek() < value:
root = s.pop()
if value < root:
return False
s.push(value)
return True
def spiral_print(root):
s1 = Stack()
s2 = Stack()
s1.push(root)
while not s1.is_empty() or not s2.is_empty():
while not s1.is_empty():
node = s1.pop()
print node,
if node.left:
s2.push(node.left)
if node.right:
s2.push(node.right)
while not s2.is_empty():
node = s2.pop()
print node,
if node.right:
s1.push(node.right)
if node.left:
s1.push(node.left)
def merge_overlapping_intervals(array):
# sort based on start time
array.sort()
s = Stack()
s.push(array[0])
for i in range(1, len(array)):
if not s.is_empty():
element = s.pop()
curr_element = array[i]
if is_overlapping(element, curr_element):
s.push(merge(element, curr_element))
else:
s.push(element)
s.push(curr_element)
print [(i.start, i.end) for i in s.items]
def print_common_nodes(root1, root2):
s1 = Stack()
s2 = Stack()
while True:
# push left nodes of trees to stack
if root1:
s1.push(root1)
root1 = root1.left
elif root2:
s2.push(root2)
root2 = root2.left
elif not s1.is_empty() and not s2.is_empty():
root1 = s1.peek()
root2 = s2.peek()
if root1.data == root2.data:
# found match, check next in both trees
print root1.data,
s1.pop()
s2.pop()
root1 = root1.right
root2 = root2.right
elif root1.data < root2.data:
# move to next node in first tree
s1.pop()
root1 = root1.right
# set this to none or in the next iteration it gets
# pushed to s2 again
root2 = None
else:
s2.pop()
root2 = root2.right
root1 = None
else:
break
def iterative_in_order(root):
current = root
s = Stack()
done = False
while not done:
if current is not None:
s.push(current)
current = current.left
else:
if not s.is_empty():
current = s.pop()
print current,
current = current.right
else:
done = True
from G4G.Problems.stack.stack import Stack
def insert_at_bottom(stack, element):
if stack.is_empty():
stack.push(element)
else:
el = stack.pop()
insert_at_bottom(stack, element)
stack.push(el)
def reverse_stack(stack):
if not stack.is_empty():
el = stack.pop()
reverse_stack(stack)
insert_at_bottom(stack, el)
if __name__ == '__main__':
s = Stack()
s.push(1)
s.push(2)
s.push(3)
reverse_stack(s)
while not s.is_empty():
print s.pop(),