Longest common prefix in python

Ruby

2 min read

·

Oct 24, 2026

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We need to find the letters that are present at the beginning of every word.

strs = [“flower”, “flow”, “flight”]

Output: “fl”

flower
flow
flight

Iteration 1:
├── f  ← common
├── l  ← common
├── o  ← common
├── w  ← common
├── e  ← common
└── r  ← common

Interation 2:
│
├── f  ← common
├── l  ← common
├── o  ← common
└── w  ← NOT common

Interation 3:
│
├── f  ← common
├── l  ← common
└── i  ← NOT common

Visual Representation of the code:

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Python Code:

class Solution:
    def longestCommonPrefix(self, strs):
# add edge case to check if the given string is empty
       if not strs:
          return ""
        
        prefix = strs[0]
        for word in strs:
            while prefix != word[:len(prefix)]:
              prefix=prefix[:-1]
#if no common prefix are found return null
              if prefix == ""
                  return ""
        return prefix

Main Part of the code:

while prefix != word[:len(prefix)]:

why do we need this in a LOOP??

Part 1: prefix

This is the part of the word we are currently checking.

flower

Our starting prefix

Part 2: word[:len(prefix)]

This takes the first few letters of the current word — the same number of letters as prefix.

For example, if prefix = "flower", its length is 6. If word = "flow", this expression takes the first 6 characters available from "flow". Since the word has only 4 characters, the result is "flow".

Part 3: !=

This means “is not equal to.”

The condition checks whether prefix and the beginning of word are different.

Why we need WHILE instead of IF?

Suppose:

• prefix = "flower"
• word = "flow"

We want to check if prefix matches the beginning of word.

Imagine that prefix is "flower" and the current word is "flight".

We need to remove several letters before finding a match:

flower →Doesn’t match flight

flowe →Doesn’t match flight

flow → Doesn’t match flight

flo → Doesn’t match flight

fl → Matches the beginning of flight

This explains -

• if checks a condition once.
• while checks the condition repeatedly and keeps running as long as the condition is true.

Here, we need while because we might need to remove one letter, two letters, or more before finding the common prefix.