读vector大小导致数组越界

第二段循环为什么到了-1还能通过，最终导致越界错误

#include<iostream>
#include<vector>
using namespace std;
int main()
{
    int k = 7;
    vector<int> cardPoints(7, 0);
    cardPoints = { 9,7,7,9,7,7,9 };
    int s = 0;
    int t = k;
    for (int i = 0; i < k; i++)
    {
        s += cardPoints[i];
    }
    int max = s;
    for (int i = cardPoints.size() - 1; i >= cardPoints.size() - k; i--)
    {
        s = s - cardPoints[--t] + cardPoints[i];
        if (max < s)
            max = s;
    }
    return 0;
}