Algorithms & data structures
Classic patterns — search, sort, stacks, queues, and practical complexity-aware code.
Count Smaller Elements to the Right in Python
Return a list where each index counts how many elements to its right are smaller than that element using a clean O(n²) nested-loop approach.
def count_smaller_elements(arr):
"""
Return a list where result[i] is the number of elements
to the right of arr[i] that are smaller than arr[i].
"""
result = []
for i in range(len(arr)):
count = 0
for j in range(i + 1, len(arr)):
if arr[j] < arr[i]:
…Find All Indices of a Target Value in a Python List
Returns a list of all indices where a given target value appears in a Python list using a list comprehension with enumerate.
def find_all_indices(arr, target):
return [i for i, value in enumerate(arr) if value == target]
if __name__ == "__main__":
sample_list = [4, 2, 7, 2, 9, 2, 1, 2]
target = 2
result = find_all_indices(sample_list, target)
print(result)Find First Duplicate Index in Python
Return the index of the first element that appears more than once in a list, using a dictionary for O(n) time.
def find_first_duplicate(arr):
seen = {}
for index, value in enumerate(arr):
if value in seen:
return index
seen[value] = index
return -1
if __name__ == "__main__":
test_array = [3, 5, 2, 8, 5, 1, 2]
result = find_first_duplicate(test_array)
print(f"Array: {test_arr…Find Maximum Distance Between Identical Elements in Python
Compute the maximum index distance between any two identical elements in a list using a dictionary to track first occurrences.
from collections import defaultdict
def max_distance_between_identical(nums):
first_occurrence = {}
max_dist = 0
for i, num in enumerate(nums):
if num in first_occurrence:
dist = i - first_occurrence[num]
max_dist = max(max_dist, dist)
else:
first_occur…Find Pivot Index in Python
Locate the index where the sum of elements to the left equals the sum to the right, using a single pass with prefix sums.
def find_pivot_index(nums):
total = sum(nums)
left_sum = 0
for i, num in enumerate(nums):
if left_sum == total - left_sum - num:
return i
left_sum += num
return -1
if __name__ == "__main__":
test_cases = [
[1, 7, 3, 6, 5, 6],
[1, 2, 3],
[2, 1, -…Find the Equilibrium Index of a List in Python
Find every index in a list where the sum of elements to its left equals the sum to its right, using a single pass.
def find_equilibrium_indexes(arr):
total = sum(arr)
left_sum = 0
indexes = []
for i, num in enumerate(arr):
total -= num
if left_sum == total:
indexes.append(i)
left_sum += num
return indexes
if __name__ == "__main__":
test = [1, 2, 3, -1, 2, 3]
result =…Find the First Index Where a Condition Is True in Python
Search any iterable for the first element matching a predicate and return its index, or -1 if none match.
def first_true_index(items, condition):
"""Return the first index where condition(item) is True, or -1 if none match."""
for i, item in enumerate(items):
if condition(item):
return i
return -1
if __name__ == "__main__":
numbers = [1, 3, 5, 8, 10, 12]
# Find first number greate…Find the Last Index Where a Condition Is True in Python
This code scans a sequence from the end and returns the index of the last element that satisfies a given condition, or -1 if none do.
def last_index_where(sequence, condition):
"""Return the index of the last element in sequence that satisfies condition."""
for i in range(len(sequence) - 1, -1, -1):
if condition(sequence[i]):
return i
return -1
if __name__ == "__main__":
numbers = [1, 4, 7, 2, 9, 5, 8, 3]
is_…Remove item at index without pop in Python
Remove an item at a given index from a list without using pop by slicing the list around the index.
def remove_at_index(lst, index):
"""Remove item at index and return the new list."""
if index < 0 or index >= len(lst):
raise IndexError("Index out of range")
return lst[:index] + lst[index + 1:]
if __name__ == "__main__":
items = [10, 20, 30, 40, 50]
result = remove_at_index(items, 2)
…Reorder a List by Odd Even Indices in Python
Splits a list into two sublists based on 1-based index parity, then concatenates odd-indexed elements before even-indexed ones.
def reorder_by_odd_even(items):
"""Reorders a list so that elements at odd indices come first,
followed by elements at even indices (1-based).
Example: [0,1,2,3,4,5,6] -> [1,3,5,0,2,4,6]
"""
odds = [items[i] for i in range(1, len(items), 2)]
evens = [items[i] for i in range(0, len(items), …Browse by section
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Algorithms & data structures — Python code examples
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