通过动态规划方式实现全排列

短命女 2022-08-05 14:15 204阅读 0赞

原文：[http://www.codeproject.com/Articles/891811/Calculating-Permutation-using-dynamic-programming][http_www.codeproject.com_Articles_891811_Calculating-Permutation-using-dynamic-programming]

The application accepts inputs from the user which is a string and will be passed as an array of chars to PermutationCalculator.In the  Calculate function because we already know how many items will be generated( n!) we specify the total item which is supposed to be returned. If you are not interested in allocating n! elements I recommend you to read Lexicographic order part from  [Counting And Listing All Permutations][] by Alexander Bogomolny.

public class PermutationCalculator
        {
            int Fact(int cnt)
            {
                int sum = 1;
                while (cnt > 0)
                {
                    sum *= cnt;
                    cnt--;
                }
                return sum;
            }
            public List<char[]> Calculate(char[] items)
            {
                if (items == null || items.Length == 0)
                    return new List<char[]>();
                int length = items.Length;
                int currentPosition = length - 1;
                int totalItem = Fact(length);
                List<char[]> currentResult = new List<char[]>(totalItem);
                List<char[]> previousResult = new List<char[]>(totalItem);
                //Add last item to the previousResult list
                previousResult.Add(new char[] { items[currentPosition] });
                while (currentPosition > 0)
                {
                    currentResult.Clear();
                    foreach (var item in previousResult)
                    {
                        currentResult.AddRange(AddItem(item, items[currentPosition - 1]));
                    }
                    if (currentPosition - 1 > 0)
                    {
                        previousResult.Clear();
                        previousResult.AddRange(currentResult);
                    }
                    currentPosition--;
                }
                return currentResult;
            }
         
            private List<char[]> AddItem(char [] currentItem, char newItem)
            {
                List<char[]> result = new List<char[]>(currentItem.Length+1);
                int pos = 0;
                while (pos <= currentItem.Length)
                {
                    char[] item = new char[currentItem.Length + 1];
                    int i = 0,j=0;
    
                    while(j<currentItem.Length+1)
                    {
                        if (j == pos)
                        {
                            item[j] = newItem;
                           
                        }
                        else
                        {
                            item[j] = currentItem[i++];
                        }
    
                        j++;
                    }
                  
                    result.Add( item);
                    pos++;
                }
                
                return result;
            }
    
            public List<char[]> CalculateParallel(char[] items)
            {
    
                if (items == null || items.Length == 0)
                    return new List<char[]>();
                int length = items.Length;
                int currentPosition = length - 1;
                int totalItem = Fact(length);
                var currentResult = new List<char[]>(totalItem);
                var previousResult = new List<char[]>(totalItem);
                int taskLength = 1000;
                previousResult.Add(new char[] { items[currentPosition] });
    
                while (currentPosition > 0)
                {
                    currentResult.Clear();
                    int count = previousResult.Count;
                    int numTask = count / taskLength;
                    if (count % taskLength != 0)
                        numTask += 1;
    
                    Task<List<char[]>>[] tasks = new Task<List<char[]>>[numTask];
                    int startTask = 0;
                    var position = currentPosition;
                    int taskInd = 0;
                    while (taskInd < numTask)
                    {
                        tasks[taskInd++] = Task.Factory.StartNew(e =>
                        {
                            int start = (int)e;
                            int end = start + taskLength;
                            if (end > previousResult.Count)
                                end = previousResult.Count;
                            var mylist = new List<char[]>((end - start) * (previousResult[0].Length + 1));
                            for (int i = start; i < end; i++)
                            {
                                mylist.AddRange(AddItem(previousResult[i], items[position - 1]));
                            }
                            return mylist;
                        }, startTask);
                        startTask += taskLength;
                    }
                    Task.WaitAll(tasks);
                    foreach (var task in tasks)
                    {
                        currentResult.AddRange(task.Result);
                        task.Dispose();
                    }
    
                    if (currentPosition - 1 > 0)
                    {
                        previousResult.Clear();
                        previousResult.AddRange(currentResult);
                    }
    
                    currentPosition--;
                }
    
                return currentResult;
            }
    
        }

eg:

static void Main(string[] args)
            {
                string inputString = Console.ReadLine();
                PermutationCalculator permutationCalculator = new PermutationCalculator();
                var result = permutationCalculator.Calculator(inputString.ToCharArray());
                Console.WriteLine("Total items:" + result.Count);
                foreach (var item in result)
                    Console.WriteLine(item);
                Console.ReadKey();
            }

作者修改过第一版后分享了修改好的博文： [http://www.codeproject.com/Tips/894053/Calculating-Permutation-without-Recursion-Part][http_www.codeproject.com_Tips_894053_Calculating-Permutation-without-Recursion-Part] (非递归方式数组排列)

[http_www.codeproject.com_Articles_891811_Calculating-Permutation-using-dynamic-programming]: http://www.codeproject.com/Articles/891811/Calculating-Permutation-using-dynamic-programming
[Counting And Listing All Permutations]: http://www.cut-the-knot.org/do_you_know/AllPerm.shtml
[http_www.codeproject.com_Tips_894053_Calculating-Permutation-without-Recursion-Part]: http://www.codeproject.com/Tips/894053/Calculating-Permutation-without-Recursion-Part