[Solved] Merge Overlapping Intervals in Python, C/C++ and Java

You have given a list of intervals. Write a program to merge overlapping intervals.

Example

Input:
Given list of intervals: [[1, 3], [2, 6], [8, 10]]

Output:
List of intervals after merging: [[1, 6], [8, 10]]

This coding challenge was asked in Biju’s coding interview.

You can watch the video for a detailed explanation. Else, you can continue with this tutorial.

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Explanation

Here intervals [1, 3] and [2, 6] overlaps. So we can merge these two intervals. The resultant interval after merging is [1, 6].

Interval [8, 10] does not overlap. So it will be as it is.

This coding challenge was asked in Biju’s interview for a software engineering job profile.

Algorithm

• Sort the intervals by their first value if it is not sorted.
• Add the first interval in the stack.
• Traverse all the elements in the given interval array by skipping the first interval. (says i as an index of the interval and arr as interval array)
  • Pop the interval from the stack (says pop_element)
  • Check if the pop_element overlaps with the arr[i] interval.
    (Write the helper function for this. This function will take the two intervals as an input and returns True if they overlap. Otherwise, return False.)
  • If overlaps, merge these two intervals and add them to the stack. Else, add arr[i] in the stack.
• Return output stack as merged intervals.

Let’s write the code for this logic.

For coding practice, I would recommend you write your own code. Whether you write it in C/C++, Java, Python, or any other programming language, That’s fine. You can always refer to the code given below.

Python Program

Prerequisite:

Intervals are represented in the form of a list. In this code, we are going to use various list methods to manipulate the intervals. Check the Python List tutorial.

Here, one of the major steps is to sort the intervals by their first value. In Python, we have the sort() method by which we can sort the intervals by their first value. Python makes the job pretty easy. Isn’t it?

Let’s see the complete program.

Code:

# Helper function to check 
# if the two intervals overlaps
def is_overlaping(a, b):
  if b[0] > a[0] and b[0] < a[1]:
    return True
  else:
    return False


# merge the intervals
def merge(arr):
  #sort the intervals by its first value
  arr.sort(key = lambda x: x[0])

  merged_list= []
  merged_list.append(arr[0])
  for i in range(1, len(arr)):
    pop_element = merged_list.pop()
    if is_overlaping(pop_element, arr[i]):
      new_element = pop_element[0], max(pop_element[1], arr[i][1])
      merged_list.append(new_element)
    else:
      merged_list.append(pop_element)
      merged_list.append(arr[i])
  return merged_list


# test the code
arr = [[1,3], [2,6], [8,10]]
print(merge(arr))

Output

[[1, 6], [8, 10]]

Merge Overlapping Intervals is one of the really interesting coding challenges. You can implement this program in any language of your choice like C/C++, Java, and Python. Let’s share your code in the comment section below.

Aniruddha Chaudhari

I am complete Python Nut, love Linux and vim as an editor. I hold a Master of Computer Science from NIT Trichy. I dabble in C/C++, Java too. I keep sharing my coding knowledge and my own experience on <b>CSEstack.org</b> portal.