Reference library

Algorithms & data structures

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

1 match
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

Browse by section

Each section groups closely related Python snippets.

Algorithms & data structures — Python code examples

What you will find here

This page collects algorithms & data structures snippets — short, copy-ready Python you can paste into our free online IDE and run without installing anything. Each sample includes a plain-English explanation and the full source code.

Samples vs tutorials and challenges

Samples are quick reference — one concept per page. For step-by-step teaching, use our Python tutorials. To test yourself, try quizzes or coding challenges. Clean up style with the Python formatter.

Related coding tools

Free online Python IDE Write and run Python in your browser — no install. Free Python code formatter Format with autopep8 — side-by-side PEP 8 cleanup. Python code samples Copy-ready snippets to study and run in the IDE.