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Merging two ordered lists efficiently in Python

To merge two lists l1, l2 in python, one can use the following

  1. Create two pointers, p1, p2
  2. Use a while loop, which is terminated, when one of the lists reaches the end
  3. Compare the entries of the list pairwise at the pointer position
    • If the entries are equal, add l1[p1] to result and increment p1 and p2 by 1
    • If l1[p1] < l2[p2], then add l1[p1] to result, then increment p1
    • If l1[p1] > l2[p2], then add l2[p2] to result, then increment p2
  4. Check, which list is not at the end and concat the tail of this list with result
In Python code, this looks the following: 
l1 = [1, 3, 5, 7, 9, 11]
l2 = [2, 4, 6]
p1 = 0
p2 = 0
result = []
# Loop over both lists until one list is at the end
while p1 < len(l1) and p2 < len(l2):
    # If the values at the positions are the same,
    # copy the value to the result
    if l1[p1] == l2[p2]:
        result.append(l1[p1])
        result[-1] = l1[p1] + l2[p2]
        p1 += 1
        p2 += 1
        # One entry is smaller, increment the smaller pointer
    elif l1[p1] < l2[p2]:
        result.append(l[p1])
        p1 += 1
    # One entry is smaller, increment the smaller pointer
    elif l1[p1] > l2[p2]:
        result.append(l2[p2])
        p2 += 1
rest = []
if p1 < len(l1):
    rest = l1[p1:len(l1)]
elif p2 < len(l2):
    rest = list2[p2:len(l2)]
result += rest
# [1, 2, 4, 5, 6, 7, 9, 11]

About the author

is an experienced Software Engineer with a Master's degree in Computer Science. He started this website in late 2015, mostly as a digital business card. He is interested in Go, Python, Ruby, SQL- and NoSQL-databases, machine learning and AI and is experienced in building scalable, distributed systems and micro-services at multiple larger and smaller companies.

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