i was trying to solve the leetcode problem sum of range minimum i did pretty normal stuff like get the previous smallest numbers index and the next smallest numbers index for each no. stored so that i could find the range length as i – pse[i] and nse[i] – i but.. its not working for [71,55,82,55]

class Solution {
public:
    int mod = 1e9 + 7;
    int sumSubarrayMins(vector<int>& arr) {
        int n = arr.size();
        vector<int> nse(n, 0), pse(n,0);
        fill(nse, pse, arr, n);
        int sum = 0;
        for(int i = 0; i<n; i++){
            sum=(sum+((nse[i]-i)*(i-pse[i])*arr[i])%mod)%mod;
        }
        return sum;
    }
    void fill(vector<int> &nse, vector<int> &pse, vector<int> &arr, int n){
        stack<pair<int,int>> st,st2;
        pse[0] = -1;
        st.push({arr[0],0});
        for(int i = 1; i<n ; i++){
            while(!st.empty()&&st.top().first>=arr[i]) st.pop();
            if(!st.empty()) pse[i] = st.top().second;
            else pse[i] = -1;
            st.push({arr[i],i});
        }
        nse[n-1] = n;
        st2.push({arr[n-1],n-1});
        for(int i = n-2; i>=0;i--){
            while(!st2.empty() && st2.top().first>=arr[i]) st2.pop();
            if(!st2.empty()) nse[i] = st2.top().second;
            else nse[i] = n;
            st2.push({arr[i],i});
        }
    }
};

`