解法1 借鉴770题的做法 用stringstream ssin

#include <iostream>
#include <sstream>
using namespace std;
int main()
{
    string str, r, s;
    getline(cin, str);
    stringstream ssin(str.substr(0, str.size() - 1));
    int cnt = 0 , max = 0;
    while(ssin >> r)
    {
        if (r.size() > max) max = r.size(), s = r;
    }
    cout << s << endl;
    return 0;
}

解法2 双指针

#include <iostream>
using namespace std;
int main()
{
    string str, r;
    getline(cin, str);
    int l = str.size() - 1, cnt = 0;
    for(int i = 0; i < l; i ++)
    {
        int j = i;
        while(j < l && str[j] != ' ') j ++;
        if(str[i] == ' ') continue;
        if(j - i > cnt) cnt = j - i, r = str.substr(i, cnt);
        i = j - 1;
    }
    cout << r << endl;
    return 0;
}