写并归排序时，提示“段错误 (核心已转储)”

参考的是这个博客，想自己实现一遍，结果不停的报错不知道为什么

#include <iostream>
#include <vector>



void merge(std::vector<int> &nums, int start, int end, std::vector<int> &res);

void mergeSort(std::vector<int> &nums, int start, int end, std::vector<int> &res) {
    if (end - start == 1) {
        if (nums[start] > nums[end]) {
            std::swap(nums[start], nums[end]);
            return;
        }
    } else if (end - start == 0) {
        return;
    } else {
        mergeSort(nums, start, start + (end - start + 1) / 2, res);
        mergeSort(nums, (end - start + 1) / 2 + start + 1, end, res);
        merge(nums, start, end, res);
    }
};

void merge(std::vector<int> &nums, int start, int end, std::vector<int> &res) {
    int leftLen = (end - start + 1) / 2 + 1;
    int leftIdx = start;
    int rightIdx = start + leftLen;
    int resIdex = start;
    while (leftIdx < start + leftLen && rightIdx < end + 1) {
        if (nums[leftIdx] <= nums[rightIdx]) {
                //运行到这里报错，我尝试了下没有超过界限啊
            std::cout << "11" << std::endl;
            res[resIdex++] = nums[leftIdx++];
        } else {
            std::cout << "12" << std::endl;
            res[resIdex++] = nums[rightIdx++];
        }
    }
    while (leftIdx < start + leftLen) {
        std::cout << "2" << std::endl;
        res[resIdex++] = nums[leftIdx++];
    }
    while (rightIdx < end + 1) {
        std::cout << "3" << std::endl;
        res[resIdex++] = nums[rightIdx++];
    }
}

int main(int argc, char const *argv[])
{
    std::vector<int> nums = {1, 5, 7, 3 ,8, 3, 5, 6, 8, 11};
    int n = nums.size();
    std::cout << "Original Nums is" << std::endl;
    for (auto num : nums) {
        std::cout << num << ",";
    }
    std::cout << std::endl;
    std::vector<int> res;
    mergeSort(nums, 0, n - 1, res);
    std::cout << "Sorted Nums is" << std::endl;
    for (auto num : res) {
        std::cout << num << ",";
    }
    std::cout << std::endl;
    return 0;
}

显示结果为：
Original Nums is
1,5,7,3,8,3,5,6,8,11,
1
5
0,2,2,0,2,0,
1
11
段错误 (核心已转储)

段错误的原因有很多，绝大多数都是指针造成的，或者在变量初始化时分配的资源太少