# Leetcode – Positions of Large Groups Solution in Python

Many algorithmic challenges test a candidate’s ability to work with sequences, whether arrays, lists, or strings. Leetcode’s “Positions of Large Groups” problem presents a paradigm that focuses on contiguous sequences with repeating characters. This article offers a detailed exploration of the problem, presents a solution approach, and showcases its Python implementation.

1. Problem Statement
2. Grasping the Problem
3. Strategy for Solving
4. Python Implementation
5. Code Analysis
6. Further Optimizations and Variations
7. Conclusion

### 1. Problem Statement

In a string s of lowercase letters, a group is defined as one or more characters that are the same and appear consecutively. We need to return the starting and ending positions of every large group. The large groups are groups that have a length of 3 or more.

### 2. Grasping the Problem

Imagine a string like abbxxxxzzy. The ‘x’ repeats 4 times consecutively, making it a “large group.” The goal is to identify such large groups and then provide their start and end positions.

### 3. Strategy for Solving

A viable approach would involve iterating through the string while keeping track of:

• The current character being examined.
• The starting position of the current group.
• The length of the current group.

When the current character doesn’t match the previous character or we reach the end of the string, we then check if the current group’s length is 3 or more. If so, we record the start and end position.

### 4. Python Implementation

Following the strategy outlined above, here’s a Python solution:

def largeGroupPositions(s):
# Initialize the start position of the group
start = 0
# List to store results
result = []

# Iterate through the string with index
for i in range(len(s)):
# If the current character doesn't match the next one or we're at the end
if i == len(s) - 1 or s[i] != s[i + 1]:
# If the length of the group is 3 or more
if i - start + 1 >= 3:
result.append([start, i])
# Update the start position for the next group
start = i + 1

return result

### 5. Code Analysis

• The variable start keeps track of the starting position of the current group.
• The list result is used to collect the positions of large groups.
• As we iterate through the string s, we look for the point where the current character does not match the next one, signaling the end of the current group.
• If the group length is 3 or more, we add its start and end positions to the result.
• We then update the start variable to the next character, marking the beginning of a new group.

### 6. Further Optimizations and Variations

1. Using a While Loop: Instead of a for loop, one could use a while loop to iterate through the string. This could be especially beneficial if one wants more manual control over the iteration process, though it might complicate the code slightly.
2. Alternative Outputs: Instead of returning the positions of large groups, one might be interested in returning the characters that form large groups, their lengths, or other related information.
3. Dynamic Group Length: Instead of a fixed threshold of 3 for a large group, the function could be extended to take an additional parameter specifying what length constitutes a “large” group.

### 7. Conclusion

The “Positions of Large Groups” problem encapsulates a foundational concept of identifying contiguous sequences in strings. Through careful iteration and group tracking, the challenge can be efficiently solved. Mastering such problems not only helps in coding interviews but also builds a foundation for more advanced textual analysis tasks. Python’s straightforward syntax and powerful standard library further simplify these tasks, turning challenges into an enjoyable learning experience.