用KMP算法找单词首次出现位置，然后在匹配时特判单词前后空格

时间复杂度$O(n)$，$n$为文章长度

C++ 代码

#include <iostream>
#include <cstring>

using namespace std;

const int N = 1e6 + 10;

int ne[N];
string s, p;
int cnt, ans[N];
bool flag = true;

void to_lower(string &s) //转小写
{
    for (char &x: s) x = tolower(x);
}

void strStr(string s, string p) //KMP,s为长串，p为短串
{
    int n = s.size(), m = p.size();
    s = ' ' + s, p = ' ' + p;

    for (int i = 2, j = 0; i <= m; i++)
    {
        while (j && p[i] != p[j + 1]) j = ne[j];
        if (p[i] == p[j + 1]) j++;
        ne[i] = j;
    }

    for (int i = 1, j = 0; i <= n; i++)
    {
        while (j && s[i] != p[j + 1]) j = ne[j];
        if (s[i] == p[j + 1]) j++;
        if (j == m) 
        {
            if (s[i + 1] != ' ') continue;//单词后空格
            if (s[i - m] != ' ') continue;//单词前空格
            ans[cnt++] = i - m;//记录单词出现位置
            flag = false;
        }
    }
}

int main()
{
    getline(cin, p);
    getline(cin, s);
    to_lower(p), to_lower(s);
    strStr(s, p);

    if (flag) printf("-1");
    else printf("%d %d\n", cnt, ans[0]);
    return 0;
}