Create nextGreaterElement.py

nextGreaterElement.py

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+# Python program to print next greater element using stack
+
+# Stack Functions to be used by printNGE()
+
+
+def createStack():
+	stack = []
+	return stack
+
+
+def isEmpty(stack):
+	return len(stack) == 0
+
+
+def push(stack, x):
+	stack.append(x)
+
+
+def pop(stack):
+	if isEmpty(stack):
+		print("Error : stack underflow")
+	else:
+		return stack.pop()
+
+
+'''prints element and NGE pair for all elements of
+arr[] '''
+
+
+def printNGE(arr):
+	s = createStack()
+	element = 0
+	next = 0
+
+	# push the first element to stack
+	push(s, arr[0])
+
+	# iterate for rest of the elements
+	for i in range(1, len(arr), 1):
+		next = arr[i]
+
+		if isEmpty(s) == False:
+
+			# if stack is not empty, then pop an element from stack
+			element = pop(s)
+
+			'''If the popped element is smaller than next, then
+				a) print the pair
+				b) keep popping while elements are smaller and
+				stack is not empty '''
+			while element < next:
+				print(str(element) + " -- " + str(next))
+				if isEmpty(s) == True:
+					break
+				element = pop(s)
+
+			'''If element is greater than next, then push
+			the element back '''
+			if element > next:
+				push(s, element)
+
+		'''push next to stack so that we can find
+		next greater for it '''
+		push(s, next)
+
+	'''After iterating over the loop, the remaining
+	elements in stack do not have the next greater
+	element, so print -1 for them '''
+
+	while isEmpty(s) == False:
+		element = pop(s)
+		next = -1
+		print(str(element) + " -- " + str(next))
+
+
+# Driver code
+arr = [11, 13, 21, 3]
+printNGE(arr)
+
+
+
+