Recursive method to achieve full permutation (C language)

Full permutation is the mathematical tool we learned in mathematics to calculate all the permutations of a group of numbers or letters .
such as ,ABCD These four letters are randomly selected from the full arrangement of three letters 24 There are so many kinds . that , How can we use program to realize it ？
Let's discuss the manual process of this problem ：
Nothing more than to see whether the current letter is desirable when taking the current letter , If it's not advisable, look at the next , If it is desirable, take out the letter .
This is very consistent with the characteristics of recursion , We only care about whether the current letter is desirable every time .
We can give an array of subscripts , The initial values of the subscript array are all assigned 0, The length of the subscript array is the same as the length of the input string . When a character in the string is selected , Then the corresponding value of this character in the subscript array is changed to 1. And store this character into the string array we want to output at last , When the last string array has enough three characters , We just let it output , This is also the condition for recursion to stop .

The code is as follows ：

#include <stdio.h>
#include <malloc.h>
#include <string.h>

#include "tool.h"

#define targetCount 3

boolean isSafe(int *arr, int index);
void completeArrangement(int *arr, int index, char *str, char *target, int temp);

void completeArrangement(int *arr, int index, char *str ,char *target, int temp) {
    
	int i;
	if (temp == 3) {
       
		printf("%s\n",target);

		return;
	}

	for (i = 0; i < strlen(str); i++) {
    
		if (isSafe(arr,i)) {
    
			arr[i] = 1;
			target[temp] = str[i];
			completeArrangement(arr, index+1, str , target, temp+1);
			arr[i] = 0;
		}
	}
}

boolean isSafe(int *arr, int index) {
    
	if (arr[index] == 1) {
    
		return FALSE;
	} else {
    
		return TRUE;
	}
}

int main() {
    
	char str[5];
	int length;
	int *arr = NULL;
	int temp = 0;
	char  target[targetCount];
	printf("please input your string:");
	gets(str);

	length = strlen(str);
	arr = (int *) calloc(sizeof(int),length);
	completeArrangement(arr, 0, str , target, temp);

	free(arr);

	return 0;
}

#ifndef _TOOL_H_
#define _TOOL_H_

typedef unsigned char boolean;

#define TRUE 1
#define FALSE 0
#define NOT_FOUND -1


#endif

About recursion ：

Many people think recursion is very tall , It is difficult to , In fact, recursion is the repeated invocation of a method , Don't think too much when writing recursive programs , The simpler and purer the consideration, the better , And when writing recursion, don't start with the core part , contrary , When writing recursive programs, you must be very clear about the end conditions of recursion , Otherwise, it is easy to cause system stack overflow when the program is running .

Writing is not good , Please correct me .