PTA刷题C语言编程顺序颠倒输出实现

#include <iOStream>
#include <cstring>
int main(void)
{
    int a = 0;
    char ch;
    char arr1[500001];//输入数组
    char arr2[500001];//输出数组
    while ((ch = getchar()) != '\n')//读入字符串
    {
        arr1[a++] = ch;
    }
    arr1[a] = '\0';
    int len = strlen(arr1);
    int i = 0;
    int dest = len - 1;//从字符串尾部开始遍历
    while (dest >= 0)
    {
        if (arr1[dest] == ' ' && arr1[dest + 1] != ' ' && arr1[dest + 1] != '\0' || (dest == 0 && arr1[0] != ' '))//元素为单词的条件
        {
            if (i != 0)
                arr2[i++] = ' ';
            int last = dest + 1;
            if (dest == 0 && arr1[0] != ' ')//满足条件，改变last的值
            {
                last = dest;
            }
            while (arr1[last] != ' ' && arr1[last] != '\0')//将单词拷贝到输出数组中
            {
                arr2[i++] = arr1[last++];
            }
        }
        dest--;
    }
    arr2[i] = '\0';//字符串结束
    std::cout << arr2 << std::endl;
    return 0;
}

arr1[dest] == ' ' && arr1[dest + 1] != ' '&&arr1[dest+1]!='\0'

(dest == 0 && arr1[0] != ' ')

while (arr1[last] != ' ' && arr1[last] != '\0')
            {
                arr2[i++] = arr1[last++];
            }

if(i!=0)
  arr2[i++] = ' ';