runtime error: reference binding to null pointer of type 'int' (stl_vector.h) : LeetoCode 907

Question

I am trying 907. Sum of Subarray Minimums on Leetcode.

I keep getting this error:

Line 1034: Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:9

Here is what I tried (I understand it is very naive and will most probably TLE, but I would also like to know what is wrong such that it doesnt work) :-

class Solution {
public:
    int sumSubarrayMins(vector<int>& arr) {
        vector<int> ans;
        int sum =0; 
        int mn=0;
        int m = 1000000007;
        int n = arr.size();
        for(int i = 0 ; i<n; i++){
            for(int j = i; j<n; j++){
                for(int k = i; k<j; k++){
                    ans.push_back(arr[k]);
                }
                sort(ans.begin(), ans.end());
                mn = ans[0];
                sum = sum + mn;
                ans.clear();
            }
        }
        return sum%m;
    }
};

What is happening?

Answer 1

On the first iteration,

• ans has no elements
• i = 0
• j = i (j = 0)

Then no elements will be pushed to ans because i < j is false.

Therefore, mn = ans[0]; is invalid out-of-range access because ans still has no elements.