去重之后还剩几个

运行下面这段程序：

#include <algorithm>
#include <iostream>
#include <map>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;

int main() {
    vector<string> words = {"苹果", "香蕉", "苹果", "梨", "香蕉", "苹果"};
    unordered_set<string> seen;
    vector<string> out;
    for (const string& w : words) {
        if (!seen.count(w)) {
            seen.insert(w);
            out.push_back(w);
        }
    }
    cout << out.size() << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）