Permutation of strings [algorithm]__ algorithm

transferred from: http://zhedahht.blog.163.com/blog/static/254111742007499363479/

Author: Huang
originating from: http://zhedahht.blog.163.com/

Title: Enter a string to print out all the permutations of the characters in the string. For example, the input string abc, the output of the characters A, B, C can be arranged out of all the strings of ABC, ACB, BAC, BCA, Cab and CBA.

Analysis: This is a good test of recursive understanding of the programming problem, so in the past year has been frequent in the major companies in the interview, written questions.

We use the three-character ABC example to analyze the process of finding string permutations. First we fix the first character a, and we ask for the two-character BC arrangement. When the two-character BC arrangement is made, we swap the first character A with the second B to get the BAC, and then we fix the first character B and then the two-character AC arrangement. Now it's time to put C in the first place. Remember that we have previously exchanged the first character A and the second B, in order to ensure that this time C is still in the first place with the A exchange, we have C and the first character exchange before the first to Exchange B and a back. After exchanging B and a, take C and a in the first position to exchange, get CBA. We fixed the first character C again, and we asked for the arrangement of the next two characters, B, a.

Now that we know how to ask for three-character permutations, it is a typical recursive idea to ask for the next two-character arrangement after the first character is fixed.

Based on the previous analysis, we can get the following reference code:

void permutation (char* pStr, char* pbegin);

/////////////////////////////////////////////////////////////////////////
Get the permutation of a string,
For example, input string abc, it permutation is
ABC ACB BAC BCA CBA cab
/////////////////////////////////////////////////////////////////////////
void permutation (char* pStr)
{
Permutation (pStr, pStr);
}

/////////////////////////////////////////////////////////////////////////
Print the permutation of a string,
Input:pstr-input string
Pbegin-points to the begin char of string
Which we want to permutate in this recursion
/////////////////////////////////////////////////////////////////////////
void permutation (char* pStr, char* pbegin)
{
if (!pstr | |!pbegin)
Return

If Pbegin points to the end of string,
This round of permutation is finished,
Print the permuted string
if (*pbegin = = ' + ')
{
printf ("%s\n", pStr);
}
Otherwise, permute string
Else
{
for (char* pch = Pbegin; *pch! = '); + pch)
{
Swap pCh and Pbegin
char temp = *pch;
*pch = *pbegin;
*pbegin = temp;

Permutation (pStr, Pbegin + 1);

Restore PCh and Pbegin
temp = *pch;
*pch = *pbegin;
*pbegin = temp;
}
}
}

Extension 1: What if you are not all permutations of a character, but rather all combinations of characters? When the input string contains the same string, the same character exchange position is arranged in a different arrangement, but the same combination. For example, if you enter ABC, its combination is a, B, C, AB, AC, BC, ABC.

Extension 2: Enter an array of 8 numbers to determine if it is possible to place the 8 numbers on the 8 vertices of the cube, so that the three vertices on the opposite sides of the 4 group are equal to each other on the cube.

This article has been included in the "sword refers to offer--name Enterprise interviewer explaining typical programming problem" a book, there are changes, the analysis of the book in more detail. Welcome attention.

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