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Algorithms & data structures

Classic patterns — search, sort, stacks, queues, and practical complexity-aware code.

3 matches

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…

12 0 Open

Algorithms & data structures easy

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.

search list reverse

Python

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_…

12 0 Open

Algorithms & data structures medium

How to Find the Previous Smaller Element in Python

Use a monotonic stack to find the nearest smaller element to the left of each item in a list, returning -1 when none exists.

monotonic stack stack arrays

Python

from collections import deque

def previous_smaller_elements(arr):
    stack = deque()
    result = [-1] * len(arr)

    for i in range(len(arr)):
        while stack and arr[stack[-1]] >= arr[i]:
            stack.pop()
        if stack:
            result[i] = arr[stack[-1]]
        stack.append(i)

    return resul…

15 0 Open

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