/* * File: permute.c * --------------- * This file implements and tests the ListPermutation function. */#include <stdio.h>#include "genlib.h"#include "strlib.h"#include "simpio.h"/* Private function prototypes */static void ListPermutations(string str);static void PermuteWithFixedPrefix(string str, int k);static void ExchangeCharacters(string str, int p1, int p2);/* Main program */main(){    string str;    printf("This program lists all permutations of a string.\n");    printf("Enter a string: ");    str = GetLine();    ListPermutations(str);}/* * Function: ListPermutations * Usage: ListPermutations(str) * ---------------------------- * This function lists all permutations of the characters * in the string str. */static void ListPermutations(string str){    PermuteWithFixedPrefix(str, 0);}/* * Function: PermuteWithFixedPrefix * Usage: PermuteWithFixedPrefix(str, k); * -------------------------------------- * This function implements the recursive permutation algorithm. * In English, this function corresponds to the imperative * statement: Generate and print all permutations of the string * str, holding the first k character fixed.  The recursive insight * is that the permutations of n characters consist of each of * those characters followed by all permutations of the remaining * n-1 characters. */static void PermuteWithFixedPrefix(string str, int k){    int i;    if (k == StringLength(str)) {        printf("%s\n", str);    } else {        for (i = k; i < StringLength(str); i++) {            ExchangeCharacters(str, k, i);            PermuteWithFixedPrefix(str, k + 1);            ExchangeCharacters(str, k, i);        }    }}/* * Function: ExchangeCharacters * Usage: ExchangeCharacters(str, p1, p2); * --------------------------------------- * This function exchanges the characters at positions p1 * and p2 of the string s. */static void ExchangeCharacters(string str, int p1, int p2){    char tmp;    tmp = str[p1];    str[p1] = str[p2];    str[p2] = tmp;}