# Questions tagged [backtracking]

Backtracking is a general algorithm for finding solutions to some computational problem, that incrementally builds candidates to the solutions, and rejects continued processing of tracks that would lead to impossible solutions.

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### Solving a sudoku with backtracking

I wrote this java Class to solve a standard 9x9 sudoku board. It uses backtracking to solve each field of the board. I am really interested in feedback for the "isValid" and "...
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### Putting as many ships on a square board as possible

Github repository Origanally I wrote a program that let you play battleship against the computer. Pretty easy: create a number of ships generate random cells for all the ships let the user try to ...
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### Sudoku Solver using rules of Sudoku and backtracking

This code tries to solve the sudoku board using the rules of sudoku. When it does not make progress in solving, it assumes a cell value and tries again. Please review my code and help me understand ...
70 views

### Python sudoku solver using backtrack algorithm

My first attempt at a sudoku solver using the backtrack algorithm. ...
33 views

### Time and Space Complexity of Leetcode Problem #31. Next Permutation

The question is as follows: Given a collection of distinct integers, return all possible permutations. Example: ...
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### Sudoku solver using backtracting and depth-first-search

I've recently finished my first ever C program. I implemented a Sudoku solver that uses backtracking. Does my code look like a proper C-program? Are there any recommendations for improving my code? <...
154 views

### Sudoku-Solver with GUI - follow-up

Background A few months ago, I asked this and this question on my implementation of a Sudoku-Solver. I now tried to further improve this little project. Changes Minor GUI-changes Input validation "...
162 views

### Go function to generate permutations of a given integer array

I'm writing a function to generate a permutation of given []int: ...
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### Filter out unwanted substrings while generating all possible relevant substrings

Write a Program that determines where to add periods to a decimal string so that the resulting string is a valid IP address. There may be more than one valid IP address corresponding to a string, ...
257 views

### Given 2 numbers a and b find the smallest number greater than b by interchanging the digits of a and if not possible print -1

I have made the solution for this question it is based on backtracking. Can anyone suggest me how to make it work faster, I haven't practiced backtracking that much, although my solution seems to work....
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### Performance is too low, online compiler wants a faster way to find all possible combinations of k numbers out of 1…n

I have to find a method (using backtracking) to list all possible combinations, in lexicographical order,of k numbers out of the interval [1,n].. duplicates are not allowed. i.e: input: 5 3output:1 2 ...
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### N-Queens Puzzle

The N-Queens puzzle can be solved in many ways. One is by a depth-first-search backtracking algorithm. Below is my generalized version using mutual recursion: ...
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### C backtracking Sudoku solver

I wrote this program in C to solve a given Sudoku puzzle (represented as 2D array) using backtracking algorithm. How can I make it more efficient, maybe faster and more C-onic? This is my first post ...
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### Speeding up naive backtracking Sudoku Solver in Haskell

After watching a recent computerphile video on building a very simple sudoku solver I tried to implement the same in Haskell. From this CR question I learned that it is probably a better idea to use ...
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### Two versions of Word Break II - exhaustive combinations

I am solving Word Break II from leetcode: Given a non-empty string s and a dictionary wordDict containing a list of non-empty ...
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### Improved Backtracking with nqueen

I am implementing conflict-based backjumping with nqueen. I want to optimize my code especially in recursive call. In short,backjumping is similar to backtracking and it uses conflict set. When ...
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### n-Queens backtracking code

I was learning backtracking algorithms earlier today, and was excited and wrote this code for n-Queens problem. Being my first try at backtracking algorithms, I would appreciate if you guys could chip ...
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### Sudoku solver iterating too much

I need help reducing the number of iterations my current implementation takes to solve the puzzle. I've got a pre-built class I am attempting to implement, albeit I think my backtracking needs ...
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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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### How to make my backtraching sudoku solving algorithm in Python faster?

I wrote a backtracking Sudoku solving algorithm in Python. It solves a 2D array like this (zero means "empty field"): ...
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### LeetCode #37: Sudoku as Exact Cover Problem solved using Dancing Links

Introduction I provide a regular 9x9 Sudoku solver, reducing the puzzle to an Exact Cover Problem. The algorithm used is Knuth'...
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I solved the following problem using backtracking: We are given a list of military units, their weight and their (numerical) strength. We have a number of ships with a limited carrying capacity....
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### Find number of islands Leetcode

Here is the question: Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally ...
683 views

### Java Magic square program

Here is my improved version of my Magic square program from following this earlier version. I also added a few comments. Any help for improvements would be really appreciated. ...
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### Speed up Magic square program

I have written a Java program to calculate magic squares with recursion and backtracking. A 3*3 Magic square is calculated in about 1 sec, but a 4*4 needs about 50 minutes on my laptop with Intel i5. ...
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### N Queens in Python using backtracking

I write this solution to the popular N Queens problem using backtracking algorithm. I am relatively new to Python. I would like to know what are the ways to refactor this code and also code style of ...
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### N Queen Problem C++

Following code is solving n queen problem with c++ using backtracking. I saw other people solution they are very short like 20 to 30 lines. Is there way improve following code? ...
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### C++ code for solving a sudoku using backtracking

I made a sudoku solver using backtracking in C++ and I would like to know what can I do to speed up my code. I am new to this language and I don't know all its special tricks yet! I was told to go to ...
427 views

### Backtracking solver for n queens problem and knight's tour

Playing with https://ocaml.org/learn/tutorials/99problems.html#Miscellaneous-Problems I wrote a backtracking solver for the n queens problem, then the knight's tour, and realised I could generalise ...
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### N Queens in Python

I used Python very occasionally, today I solved n queens in Python just for fun. I am sure there are better way of doing certain things in Python and can't wait for improvement suggestions! ...
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### Recover flights from found tickets

I just found a whole bunch of plane tickets that only have a start and destination on them. I also know the route started in a certain city. I would like to recover the (alphabetically) first possible ...
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### 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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### Beginner's Sudoku solver in C

I'm beginner programmer and I've challenged myself to write sudoku solver in C. I have finished it in few days and now I want to make it faster. With this sample input execution time is 1.781 ...
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### Backtracking maze traversal

I have a functional backtracking solution, but I am only able to finish 22/25 test cases before exceeding my specified time limit. Any suggestions on how to improve the speed are greatly appreciated! ...
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### Ruby Sudoku Solver without classes

I'm writing a Sudoku solver in Ruby to complete a coding challenge with the following restrictions. Failing to follow the following restrictions will result in an invalid submission: Do ...
326 views

### Puzzle game solver via Backtracking

I'm relatively new to Haskell and I'd like to get feedback on the style of my program. Specifically: Coding Style: Can any parts be written in a more concise or more readable way? Misuse: Are there ...
527 views

### N queens variation recursion Python 3

I am trying to solve a modified version of the N-queens problem, where the addition is that for any NxN board (3<=N<=12) there are M holes (0<=M<=N^2) that are placed at random (but given) ...
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### Two approaches to print all permutations - returning versus passing through the “result” list

I have noticed many of the backtracking problems have two ways of solving. One is to return "whatever's the required list", vs passing-through the "result" to every call and appending to it. What is ...
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### A KenKen puzzle/solver in Python

I've written a simple KenKen puzzle/solver in Python. I'd love some feedback on the design of the puzzle, as well as the architecture for the solver. To model the puzzle, I have the following classes:...
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### Algorithm to transform one word to another through valid words

I have been practicing backtracking and I wanted to know how I can improve my code. For eg, I don't want to use global. Also, I am not sure if my code will work for all the cases. ...
196 views

### Google Code Jam 2017 1B - Stable Neigh-bors

Last week I participated in Google code Jam round 2B and miserably failed. I still decided to finish the second question, for practice. Here it is: Problem You are lucky enough to own N pet ...
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### Leetcode 17. Letter Combinations of a Phone Number

Problem Statement Given a digit string, return all possible letter combinations that the number could represent. A mapping of digit to letters (just like on the telephone buttons) is given below. ...
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### Problems with backtracking algorithm in C++

I have to implement an algorithm that generates a modified version of a Thue Sequence, being that there can't be any adjacent sub-sequence can repeat itself using the letters N,P and O. For example, ...
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### Coloring grid game

I have grid N × M in which each cell is coloured with one colour. When the player clicks on any cell of the grid of colour α, the cell in the top-leftmost corner of the grid, of colour β, receives ...
741 views

### UVA 750: 8 Queens Chess

In chess it is possible to place eight queens on the board so that no one queen can be taken by any other. Write a program that will determine all such possible arrangements for eight queens given ...
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### Knight's tour code

Another one for some late night snack. - Knight's tour! Please provide your suggestions/flaws/optimization to this knight's tour backtracking code.. ...
738 views

### Partitions of a number (backtracking) - time efficiency

I had to write an algorithm that prints the partitions of a natural number in lexicographic order. Example of I/O: If the algorithm reads from ...