# Questions tagged [recursion]

Recursion in computer science is a method of problem solving where the solution to a problem depends on solutions to smaller instances of the same problem.

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### Find the minimum number of coin change

Description: You are given coins of different denominations and a total amount of money amount. Write a function to compute the fewest number of coins that you need to make up that amount. If that ...
336 views

### Recursive solution for Longest Substring Without Repeating Characters written in Elixir

Here's my recursive implementation of a sliding window algorithm to find the longest substring of a given string without repeating characters. Is there a more idiomatic-Elixir way to write this? <...
152 views

### Merging bin-data via a bin count threshold

When performing a chi-squared test, one takes the square of the differences of the expected counts per bin and observed counts per bin, and divides these per-bin differences by the expected counts per ...
615 views

### Find subsets of size K in N set

The question is: There is a set An, and it consists of integers from 1 to n. ...
365 views

### Recursive Fibonacci in Java

I just started learning recursion and memoization, so I decided to give a simple implementation a shot. Works with some simple test cases I came up with. Is there anything I can add to make this more ...
1k views

### Sum numbers represented as linked lists

Problem Description: You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add ...
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### Eight Queens Recursive

I am working on a Java textbook chapter on recursion, and I am to implement an Eight Queens solution using recursion. Though I know that I am to post working code in Code Review, I do believe this ...
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### Finding non-self-intersecting paths of certain moves that touch all points in a grid

The problem I'm trying to solve is the following: You have a 10x10 grid and starting from the upper-left point you have to touch all the points, without passing through the points that have already ...
657 views

### Python Converting numbers to words, using Recursive

My function recursively calls itself until it decodes every numbers. rec_ntw face_value_count -> (Incremental variable) denotes ones, tens, hundreds and thousands face_value -> (Dictionary) denotes ...
849 views

### Max Sum of Nodes in Each Path in Binary Tree

I wanted to get a review on an algorithm I wrote for a binary tree problem. The problem is the following. Return the maximum sum between all branches in a binary tree. A branch is defined as all ...
6k views

### A beautiful and easy way of permutation with recursion

I want to share a better version of permutation with you. Yesterday, when I was reading Horowitz's Fundamentals of Data Structures In C++, and I noticed that the result of the code, at page 32, in ...
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### Reverse a string recursively

I just finished this exercise where i should reverse a string with a recursive function. First of all, i created a separate function called swap to swap the chars, should i write it as function ...
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### CTE Query and DateTime Matching

I have two tables (they are defined below and you can use the SQL below to build them) ...
1k views

### Find the longest path in a matrix where each step has entries that differ by 1

Given an N*N matrix that all numbers are distinct in it, the function should find the maximum length path (starting from any cell) such that all the cells along the path are in increasing order with a ...
152 views

### Ackermann function in Rust

Using the algorithm at the end of this research paper, I've written an extremely fast 1 Ackermann function in Rust. Can the implementation be made faster? Thoughts: Using a struct makes a lot of ...
62 views

### Simple backtracking code

Is this good or can it be improved? The program should backtrack the two variables until this logic operation is true: a & b. ...
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### C# recursive backtracker labyrinth generation & game

So for over a weekend I decided to write something rather simple. For maze generation I've followed this algorithm and aside from that I've made a little game, to go from A to B. Everything works fine ...
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### Apply arbitrary action during recursive generator function

I've got this recursive generator function. It will traverse an object looking for "leafs", any keys that don't point to sub-objects. Then it applies one of two actions on the leaf and yields it. Any ...
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### Check the equality for each subset of a string S against the target T and return a count

I wrote the following code to solve this leetcode problem: ...
57 views

### Make a given number by adding given numbers

I have created a code that checks if a number can be made by adding different quantities of given numbers and if so returns the quantities. Is this the best way to do this or is there a more efficient ...
104 views

### Recursion memorization table

The following code is for this problem. Input The input consists of a single line with three integers n , s , and k (1 ≤ n ≤ 10,000, 1≤ k ≤ s ≤ 500 ). n is the number of throws, k ...
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### AWS EC2 metadata fetcher in Python

Update https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html This seems to be the schema for the instance metadata, though the way they describe it isn't really handy for ...
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### Maze maker for game using recursive method

I wold like some feedback on this mazefactory, it creatse a maze in a 2D array starting in the middle (the treasure room) and creating new rooms until it reaches the outside of the maze. The maze is ...
505 views

### Recursive function challenge

Problem statement Given a list of float numbers, and four operators +, -, *, / with flat preference, find the maximum value by inserting operator between each consecutive pair of numbers. For ...
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### Improving Fibonacci recursion with BigIntegers

I've been tasked with making a fast Fibonacci recursive method that uses BigInteger to calculate REALLY big numbers. However, to calculate numbers past 30 it takes ...
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### Recursive grid traveling algorithm

I'm trying to come up with a solution to a coding challenge on Kattis, where there is a 2D grid filled with x's and o's, and the goal is to figure out whether or not a given starting coordinate can ...
1k views

### Tower Hopper problem recursive approach

The Tower Hopper problem gives us an array of values representing heights that express how far we can jump from a certain tower, and asks whether there's a way to get from ...
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### Recursive matrix multiplication

I've just written a recursive matrix multiplication and I want to know my mistakes or problems. Is there a better way doing this? ...
129 views

### Maximum sub array sum equal to k

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k. For example: Input: nums = [1,1,1], k = 2 Output: 2 ...
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### Insertion Sort Algorithm

Here is my insertion sort algorithm: Note: Insertion sort works by starting off with the first item in the list and then inserting the next item in the original list into the correct place in the ...
305 views

### Reinterpreting a string, treating < as a backspace character

I've come up with a solution to this coding challenge using recursion. In summary, the input is a string where the < symbol represents a backspace (up to 1 ...
184 views

### Advent of Code 2017, Day 7 - Recursive Circus

This is my solution to Advent of Code 2017, Day 7 – Recursive Circus (both parts, you might not be able to see the second part unless you're logged in and solved the first part, but I'll summarize it ...
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### Program that should output nth arrangement of paintings

I'm currently practicing for the British informatics olympiad by doing past papers. I'm on question 3 of the 2015 paper and trying to use recursion to solve it. I'm pretty new to using recursion but ...
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### AoC Day 24 2017: Electromagnetic Moat

This year's AoC has a puzzle for day 24 that was (supposedly) intended to be solved with recursion. Here is the description: --- Day 24: Electromagnetic Moat --- The CPU itself is a large, ...
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### Fibonacci in linear time by using an extra pointer

I have a function to find the nth number in a Fibonacci sequence, in which I am recursively calling the function. The sum is stored in a class variable and I have an extra pointer I increment every ...
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### binary search tree Inorder successor to test candidate confidence

Problem statement Given the root node and a node in the binary search tree, find the inorder successor of the given node. Here is the example of binary search tree, Given the node with ...
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### Python 3 - Fibonacci Implementation

I wrote a function returning the n-th Fibonacci number in Python 3: ...
810 views

### Finding Second Largest Element in an Array

Here's my implementation using divide and conquer in C++. What do you think of this implementation regarding running time ? ...
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### Base 26 (letters) and Base 10 using recursion

I was messing around with base conversion between decimal and Base26 (alphabet) with no 0th case because I needed it for the Google Sheets API in order to find specific cell values from a 3 ...
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### Recursive quicksort in Python

I want to find a more elegant way to build a recursive sorting function. The function randomly selects one of the numbers from a given list, k, and splits the list ...
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### Solving alethic modal logic formulas using truth trees

Disclaimer: this first section is about alethic modal logic and truth trees. It's important to the question but significant parts of the code can probably be reviewed without it. You're welcome to ...
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### Recursive BFS solution for tree traversal

I'm somewhat new to the wonderful world of Python, outside of simple codecademy-like challenges online. This function was part of a project for school. It's tested and works, but I'm looking for some ...
157 views

I've made a Ruby linked list data structure class and a lookup method. I know that lookup will only fetch the first occurrence ...
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### Segementing a recursive tree in haskell

Could somebody help me work out how to abstract the recursion from this code for me please? I'm trying to write something that takes a pandoc structure, and subdivides the tree whenever it finds a ...
168 views

THIS is the link to the problem on SPOJ. I thought it would be better to give the link to the problem instead of me trying to explain as the question is not very clear at first sight. I was able to ...
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### Function to build a string based on the recursion depth

I was doing a JavaScript challenge to test myself and I succeeded in finding a solution but the code is very ugly. I was hoping to get some opinions on what a better solution would look like. The ...
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### Given a string of letters, output all English words formable using only those letters

As an introduction to Python classes and an exercise in recursion / tree-traversal, I wrote a small script that forms all English words using only the letters in an input string. Useful for things ...
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### C# MVC Generic Custom Model Binder

I recently created a custom model binder as a training exercise and wanted to see how I could improve it. It currently uses recursion and works great, but I was wondering how I could convert this to ...
Array A contains the elements, $A_1,A_2, \ldots, A_N$. And array B contains the elements, $B_1,B_2, \ldots, B_N$. There is a relationship between $A_i$ and $B_i$: any element $A_i$ ...