[编程入门]自定义函数之字符串连接

#include<string.h>
int main()
{
  char str1[100],str2[100];
  gets(str1);
  gets(str2);
  strcat(str1,str2);
  puts(str1);
}

为什么我的码在其他环境里试了一下可以，但是报错非法访问www
#include <stdio.h>
#include <stdlib.h>
int main() {
  char *temp1 = (char *)malloc(100 * sizeof(char));
  char *temp2 = (char *)malloc(100 * sizeof(char));
  char *temp3 = (char *)malloc(100 * sizeof(char));
  int i = 0;
  char ch;
  while ((ch = getchar()) != '\n') {
    temp1[i] = ch;
    i++;
  }
  int j = 0;
  char ch2;
  while ((ch2 = getchar()) != '\n') {
    temp2[j] = ch2;
    j++;
  }
  for (int k = 0; k < i+j; k++) {
    if(k<i)
    temp3[k] = temp1[k];
    else
      temp3[k] = temp2[k-i];
    printf("%c", temp3[k]);
  }

  free(temp1);
  free(temp2);

void Connect(char a[50], char b[50])
{
	char str[100] = {0};
	int len1 = strlen(a);
	int	len2 = strlen(b);
	for (int i = 0; i < len1; i++)
	{
		str[i] = a[i];
	}
	for (int j = 0; j < len2; j++)
	{
		str[len1 + j] = b[j];
	}
	int cnt=0;
	while (str[cnt]!='\0')
	{
		printf("%c", str[cnt]);
		cnt++;
	}
}
int main()
{
	char str1[50],str2[50];
	gets(str1);
	gets(str2);
	Connect(str1, str2);
	return 0;
}

为什么我的代码不够优雅？

#include <stdio.h>
#include <string.h>
int main()
{
    char a[10],b[10];
    scanf("%s%s",a,b);
    printf("%s%s",a,b);
    return 0;
}