how to type Permutation for a given string in C# [duplicate]

问题

Possible Duplicate:
How to type sets for a given string

I'm trying to type the permutation for a string for example the string "123" should give me 123 132 213 231 321 312 I need help to fix my code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TestAAD
{
class Program
{
    static int len = 0;
    static int lenPerm = 0;
    private static void Permutations(string str, string perm , int i)
    {
        if (i == len) Console.WriteLine(perm);
        for (int k = 0; k < len; k++)
        {
             lenPerm = perm.Length;
            for (int j = 0; j < lenPerm; j++)
            {
                if ((perm[j] == str[(len - 1) - k]) && (len-1-k>0))
                    k++;
            }
            if((len-1-k) >=0)
            Permutations(str, perm + str[(len - 1) - k], i++);
        }
    }
    static void Main(string[] args)
    {
        string st = "123";
        len = st.Length;
        Permutations(st, "",0);
    }
}
}

Please if anyone can help me , Please thanks all

回答1:

Permutations are very easy to do.

public string[] FindPermutations(string word)
{
    if (word.Length == 2)
    {
        char[] c = word.ToCharArray();
        string s = new string(new[] { c[1], c[0] });
        return new[]
            {
                word,
                s
            };
    }

    List<string> result = new List<string>();

    string[] subsetPermutations = FindPermutations(word.Substring(1));
    char firstChar = word[0];
    foreach (string temp in subsetPermutations
                    .Select(s => firstChar.ToString(CultureInfo.InvariantCulture) + s))
    {
        result.Add(temp);
        char[] chars = temp.ToCharArray();
        for (int i = 0; i < temp.Length - 1; i++)
        {
            char t = chars[i];
            chars[i] = chars[i + 1];
            chars[i + 1] = t;
            string s2 = new string(chars);
            result.Add(s2);
        }
    }
    return result.ToArray();
}

回答2:

Permutations calls Permutations again in a loop len times. This second Permutations call calls Permutations again in a loop len times. This happens again and again endlessly, or at least until you get a stack overflow.

When you use recursive calls, you always must make sure that the recursion stops somewhere.

If (job not done) {
    make recursive call
}

The recursive call must make a step towards the completion of the job, otherwise it will never finish.

UPDATE 1 (only addressing the exception problem)

The method is difficult to read. Instead of repeating the expression len-1-k several times, reverse the loop! for (int k = str.Length - 1; k >= 0; k--) and decrease k with k-- instead in the inner loop. I also got rid of the variable len which is superfluous.

In your implementation len-1-k is always >= 0. Therefore the recursive calls will never end. Therefore I changed the condition for decreasing k. It will decrease k even if it is already 0. In order not the get an index out of bounds error in str[k] the condition k >= 0 must be checked first.

private static void Permutations(string str, string perm, int i)
{
    if (i == str.Length)
        Console.WriteLine(perm);
    for (int k = str.Length - 1; k >= 0; k--) {
        int lenPerm = perm.Length;
        for (int j = 0; j < lenPerm; j++) {
            if (k >= 0 && perm[j] == str[k])
                k--;
        }
        if (k >= 0)
            Permutations(str, perm + str[k], i++);
    }
}

public static void Start()
{
    string st = "123";
    Permutations(st, "", 0);
}

This doesn't produce an endless recursion any more but the result is still not correct. I let you figure out how to improve the code further.

UPDATE 2 (creating correct permutations)

Finally here is my working implementation

public static class PermutationBuilder
{
    private static char[] _characters;
    private static List<string> _list;

    public static IEnumerable<string> GetPermutations(string characters)
    {
        _characters = characters.ToCharArray();
        _list = new List<string>();
        AddPermutations("", 0);
        return _list;
    }

    private static void AddPermutations(string permutation, int level)
    {
        if (level >= _characters.Length) {
            _list.Add(permutation);
        } else {
            for (int i = 0; i < _characters.Length; i++) {
                char ch = _characters[i];
                if (ch != ' ') {
                    _characters[i] = ' ';
                    AddPermutations(permutation + ch, level + 1);
                    _characters[i] = ch;
                }
            }
        }
    }
}

Note that I temporarily mark the characters that have been used with a space character.

You call it like this

foreach (string permutation in PermutationBuilder.GetPermutations("123")) {
    Console.WriteLine(permutation);
}

来源：https://stackoverflow.com/questions/13779030/how-to-type-permutation-for-a-given-string-in-c-sharp

标签

recursion

permutation

backtracking