reference binding to null pointer error during vector insertion

Question

I am trying to solve a particular leetcode problem but I can't seem to figure out where I am going wrong.

The question is this one:https://leetcode.com/problems/group-anagrams/ and my code is:

class Solution {
public:
    vector<vector<string>> groupAnagrams(vector<string>& str) {
        unordered_map<string, vector<int>> map;
        vector<string> newstr = str;
        int size = str.size();
        //pushing those indices of `str` into the map which share common permutation
        for(int i=0; i<size; i++){
            sort(newstr[i].begin(), newstr[i].end());
            map[newstr[i]].push_back(i);
        }

        vector<vector<string>> ans;
        int i=0;
        for(auto itr: map){
            int v_size = itr.second.size();
            for(int j=0; j<v_size; j++)
                //cout<<str[itr.second[j]]<<" "; //this query works perfectly
                ans[i].push_back(str[itr.second[j]]); //erroneous code
            i++;
        }
        return vector<vector<string>>();
        // return ans;
    }
};

Answer 1

You are accessing ans[i] without allocating any memory in ans beforehand. This creates a data access that is out of the vector's bounds (0). If you meant to add a new vector to ans, use ans.push_back(). As balki said, you can also remove redundant allocations by allocating all entries upon declaration, like this:

vector<vector<string>> ans { map.size() };

This will make your code work without any additional changes needed (as far as I can see).