`sorted()`

Introduction

Python has two ways to sort data: 1. list.sort(): An in-place method that modifies a list and returns None. 2. sorted(): A built-in function that takes any iterable, leaves the original alone, and returns a new sorted list.

In interviews, sorted() is used when you need to sort strings, tuples, or when you don't want to mutate the original input array.

Basic Usage

nums = [3, 1, 4, 1, 5]

# Returns a new list
ascending = sorted(nums)
print(ascending) # [1, 1, 3, 4, 5]

# The original is untouched
print(nums) # [3, 1, 4, 1, 5]

Sorting Strings

Because strings are immutable, they do not have a .sort() method. You must use sorted(). Note that sorted() always returns a list, so you must join it back into a string.

s = "cba"

# Returns a list of chars
sorted_chars = sorted(s)
print(sorted_chars) # ['a', 'b', 'c']

# Join back to a string
print("".join(sorted(s))) # "abc"

The `reverse` Argument

To sort in descending order, use reverse=True.

nums = [3, 1, 4, 1, 5]
descending = sorted(nums, reverse=True)
print(descending) # [5, 4, 3, 1, 1]

The `key` Argument (Crucial for Interviews)

Just like min() and max(), sorted() accepts a key function. This function is called on each element before making comparisons.

Sorting by Length

words = ["apple", "banana", "kiwi", "pear"]

# Sort by length
print(sorted(words, key=len))
# ['kiwi', 'pear', 'apple', 'banana']

Sorting by Multiple Criteria (Tuples)

If you return a tuple from the key function, Python sorts by the first element. If there's a tie, it looks at the second element, and so on.

students = [
    ("Alice", "B", 12),
    ("Bob", "A", 12),
    ("Charlie", "A", 10)
]

# Sort by Grade (ascending), then by Age (descending)
sorted_students = sorted(
    students, 
    key=lambda x: (x[1], -x[2])
)

print(sorted_students)
# [('Charlie', 'A', 10), ('Bob', 'A', 12), ('Alice', 'B', 12)]

(Note: Using -x[2] works for integers to reverse the sort order for just that specific field).

Time and Space Complexity

Python uses Timsort (a hybrid of Merge Sort and Insertion Sort). - Time Complexity: \(O(N \log N)\) worst/average case. \(O(N)\) best case (if already sorted). - Space Complexity: \(O(N)\) to create the new list.

Summary

Use sorted() when you need to sort strings or want to keep the original iterable intact.
Use reverse=True for descending order.
Use key=lambda x: ... for custom sorting logic.
Return a tuple from the key function to sort by multiple criteria.

sorted()