Basic example
Input
[4, 5, 2, 25]
Output
[2, 2, -1, -1]
For 4 the next smaller is 2; for 5 it's 2; for 2 none; for 25 none.
Full lesson preview
Find the next smaller element
For each element in an array, find the next smaller element to its right using a monotonic stack.
Python practice15 minStacks & QueuesIntermediateLast updated March 27, 2026
Problem statement
Given a list of integers nums, return a list result of the same length where result[i] is the next smaller element to the right of nums[i]. If no such element exists, result[i] should be -1. Aim for an O(n) time solution using a stack that maintains a monotonic increasing sequence of values (or their indices).
Task
Implement next_smaller(nums) that returns a list where each position contains the first smaller element to the right, or -1 if none.
Examples
Input format
A single list of integers, e.g. [4, 5, 2, 25].
Output format
A list of integers of same length, e.g. [2, 2, -1, -1].
Constraints
0 <= len(nums) <= 10^5; -10^9 <= nums[i] <= 10^9; expected O(n) time and O(n) extra space.
Samples
Sample 1
Input
[3, 7, 1, 7, 2]
Output
[1, 1, -1, 2, -1]
For 3 the next smaller is 1, for 7 it's 1, 1 has none, for the next 7 it's 2, for 2 none.