请看看这几句怎么理解?

while (arr1[i])i++;
    while (arr2[j])
        arr1[i++] = arr2[j++];
    arr1[i] = '\0';
    puts(arr1);

把字符串arr2添加到字符串arr1末尾，和strcat()函数功能一样

while (arr1[i])i++;         //找到arr1的末尾，即'\0'的数组下标
while (arr2[j])             //若未遍历至arr2的末尾，则执行循环
    arr1[i++] = arr2[j++];  //复制字符
arr1[i] = '\0';             //添加字符串结束符'\0'
puts(arr1);                 //输出结果