例子如下
Input
hello  ABC
Output
helloABC
main函数

#include <stdio.h>
int main() {
        void myStrcat(char[], int, char[], int);
        char firstStr[101];
        char secondStr[51];
        int firstStrLength = 0, secondStrLength = 0;
        scanf("%s %s", firstStr, secondStr);

        while (firstStr[firstStrLength++]) {}
        while (secondStr[secondStrLength++]) {}
        myStrcat(firstStr, firstStrLength-1, secondStr, secondStrLength-1);

        printf("%s\n", firstStr);

        return 0;
}

写myStrcat（）函数？

while (firstStr[firstStrLength++]) {firstStrLength ++};
while (secondStr[secondStrLength++]) { secondStrLength ++};

void myStrcat(char s1[], int s_1,char s2[], int s_2)
{
    int i=0;
    for (;i<s_2;i++)
        s1[i+s_1]=s2[i];
   
    s2[i]='\0';
}