给定一个以非降序排序的整数nums数组，查找给定目标值的开始和结束位置。
如果在数组中找不到目标，则返回[-1，-1]。
你必须编写一个运行时复杂度为O（log n）的算法。
我用leetcode写了这段代码，但它只通过了88个测试用例中的67个。有人能告诉我这段代码的问题吗？

class Solution {
public:
int FirstOccurence(vector<int> arr, int n, int x) {
    int low = 0, high = n - 1;
    int ans = -1;

    while (low <= high) {
        int mid = (low + high) / 2;
        // maybe an answer
        if (arr[mid] == x) {
            ans=mid;
            high=mid-1;
    }
    else if (arr[mid]< x) {
            low=mid+1;
    }
    else high=mid-1;
    }
    return ans;
}

int LastOccurence(vector<int> arr, int n, int x) {
    int low = 0, high = n - 1;
    int ans = -1;

    while (low <= high) {
        int mid = (low + high) / 2;
        // maybe an answer
        if (arr[mid]== x) {
            ans = mid;
            //look for smaller index on the left
            low=mid+1;
        }
        else if(arr[mid]>x){
            high=mid-1;
        }
        else {
            low = mid + 1; // look on the right
        }
    }
    return ans;
}
    vector<int> searchRange(vector<int>& nums, int target) {
        int n=nums.size()-1;
        int k=target;
        int a=FirstOccurence(nums,n,k);
    int b=LastOccurence(nums,n,k);
    return{a,b};
    }
};

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