100 Questions selected by Programmers (59)-Combination of strings [algorithm]_ sword refers to an offer

Title: Enter a string that prints all the combinations of characters in the string. For example, if you enter ABC, it has a combination of a, B, C, AB, AC, BC, ABC.

Analysis: In the 28th question of the series "string arrangement", we discussed in detail how to use recursive ideas to find the arrangement of strings. Similarly, the subject can also use recursive ideas to find the combination of strings.

Suppose we want to find a combination of M characters in a string of length n. Let's start by scanning the first character of the string. For the first character, we have two options: one is to put the character in a group, then we need to select the m-1 character in the remaining n-1, instead of putting the character in the group, we need to select the M character in the remaining n-1 characters. Both of these options are easy to implement with recursion. Here is the reference code for this idea:

void combination (char* string)
{
    if (string = = NULL) return
        ;
 
    int length = strlen (string);
    vector<char> result;
    for (int i = 1; I <= length; + + i)
    {
        combination (string, I, result)
    ;
}
 
void combination (char* string, int number, vector<char>& result)
{
    if (number = = 0)
    {
        Vector<char>::iterator iter = Result.begin ();
        for (; Iter < result.end (); + + iter)
            printf ("%c", *iter);
        printf ("\ n");
 
        return;
    }
 
    if (*string = = ' ") return
        ;
 
    Result.push_back (*string);
    Combination (string + 1, number-1, result);
    Result.pop_back ();
 
    Combination (string + 1, number, result);
}

Since a combination can be a combination of 1 characters, a character of 2 characters ... All the time to a combination of n characters, so in the function void combination (char* string) We need a For loop. In addition, we have a vector that holds the characters chosen to put into the combination.

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