vector的sort 问题

#include<iostream>#include<vector>#include<string>#include<algorithm>using namespace std;struct stu{int mark;string name;int age;};bool cmp(stu a, stu b){return (a.mark<b.mark) || (a.mark == b.mark&&a.name<b.name) || (a.mark == b.mark&&a.name==b.name&&a.age<b.age);}int main(){int n;cin >> n;vector<stu> arr(n + 1);for (int i = 0; i < n; i++){cin >> arr[i].name;cin >> arr[i].age;cin >> arr[i].mark;}sort(arr.begin(), arr.end(), cmp);for (int i = 1; i <= n; i++){cout << arr[i].name << ' ' << arr[i].age << ' ' << arr[i].mark << endl;}//while (1);return 0;}

为什么对vector执行sort之后，第0个没有元素，是从第一个开始的？