用cin读入（自动省略多余的空格）

#include <iostream>

using namespace std;

int main()
{
  string a;
  while (cin >> a) cout << a << ' ';

  return 0;
}

第二种双指针算法

#include <iostream>

using namespace std;

int main()
{
    string a;
    getline(cin, a);

    string r;
    for (int i = 0; i < a.size(); i ++ )
    if (a[i] != ' ') r += a[i];
    else 
    {
        r += ' ';
        int j = i;
        while (j < a.size() && a[j] == ' ') j ++ ;
        i = j - 1;
    }

    cout << r << endl;
    return 0;
}

局部性判断

#include <iostream>

using namespace std;

int main()
{
    string a;
    getline(cin, a);

    string r;
    for (int i = 0; i < a.size(); i ++ )
    if (a[i] != ' ') r += a[i];
    else if (a[i - 1] != ' ') r += a[i];

    cout << r << endl;
    return 0;
}