# Merge Intervals in Python

Suppose we have a collection of intervals, we have to merge all overlapping intervals. So if the intervals are like [[1,3], [2,6], [8,10], [15,18]], then the intervals after merging will be [[1,6],[8,10],[15,18]]. This is because there were two intervals those are overlapping, the intervals are [1,3] and [2,6], these are merged to [1,6]

Let us see the steps −

• if interval list length is 0, then return a blank list
• sort the interval list using quicksort mechanism
• stack := an empty stack, and insert intervals[0] into the stack
• for i in range 1 to length of intervals – 1
• last_element := top element of stack
• if end of last_element >= first element of intervals[i], then
• end of last_element = max of end of intervals[i], and end of last_element
• pop element from stack
• push last_element into stack
• else push intervals[i] into stack
• return stack

Let us see the following implementation to get better understanding −

## Example

Live Demo

class Solution(object):
def merge(self, intervals):
"""
:type intervals: List[Interval]
:rtype: List[Interval]
"""
if len(intervals) == 0:
return []
self.quicksort(intervals,0,len(intervals)-1)
#for i in intervals:
#print(i.start, i.end)
stack = []
stack.append(intervals[0])
for i in range(1,len(intervals)):
last_element= stack[len(stack)-1]
if last_element[1] >= intervals[i][0]:
last_element[1] = max(intervals[i][1],last_element[1])
stack.pop(len(stack)-1)
stack.append(last_element)
else:
stack.append(intervals[i])
return stack
def partition(self,array,start,end):
pivot_index = start
for i in range(start,end):
if array[i][0]<=array[end][0]:
array[i],array[pivot_index] =array[pivot_index],array[i]
pivot_index+=1
array[end],array[pivot_index] =array[pivot_index],array[end]
return pivot_index
def quicksort(self,array,start,end):
if start<end:
partition_index = self.partition(array,start,end)
self.quicksort(array,start,partition_index-1)
self.quicksort(array, partition_index + 1, end)
ob1 = Solution()
print(ob1.merge([[1,3],[2,6],[8,10],[15,18]]))

## Input

[[1,3],[2,6],[8,10],[15,18]]

## Output

[[1, 6], [8, 10], [15, 18]]

Updated on: 04-May-2020

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