What is causing the Stack Buffer Overflow?

Question

I was solving this question on LeetCode - https://leetcode.com/problems/k-closest-points-to-origin/ I could make two things - 1) We needed to sort the distances of given points in ascending order.
2) We also had to have the point associated with that distance from origin.

After brainstorming, I came up with the idea of using maps from c++ stl. As they would take care of sorting and also the association of distance and point. My code is as follows -

class Solution 
{
public:
    vector<vector<int>> kClosest(vector<vector<int>>& points, int k) 
    {
        map<double,vector<int>> m;
        vector<vector<int>> answer;
        for (int i = 0; i < points.size(); i++)
        {
            double x = sqrt((points[i][0] * points[i][0]) + (points[i][1] * points[i][1]));
            m.insert(pair<double, vector<int>>(x,{points[i][0], points[i][1]}));
        }
        
        for (int i = 0; i < k; i++)
        {
            auto it = m.begin();
            advance(it,i);
            answer.push_back(it->second);
        }
        
        return answer;
    }
};

It is working fine for the first 2 testcases and throwing runtime error - stack buffer overflow, I had initially used float for x but I thought because of constraints it was causing the error, so I changed the type to double but still no luck!

It would be a great help if someone can help me figure the mistake here. Thank you in advance!

Answer 1

Let's think of a case where there are two points [-1, 0] and [0, 1], and the value of k is 2. If you use map, you will get only 1 <key, value> pair for your case, because the key (in this case sqrt(1)) is same for both points. So, you need to use multimap , where you can have the same key multiple times. Read here .

The working code example based on your code:

class Solution 
{
public:
    vector<vector<int>> kClosest(vector<vector<int>>& points, int k) 
    {
        multimap<int,vector<int>> m;
        vector<vector<int>> answer;
        for (int i = 0; i < points.size(); i++)
        {
            int x = (points[i][0] * points[i][0]) + (points[i][1] * points[i][1]);
            m.insert(pair<int, vector<int>>(x,{points[i][0], points[i][1]}));
        }
        int i = 0;
        for (auto it = m.begin(); it != m.end() && i < k; it++, i++) {
            //cout << it->first << " : " << it->second[0] << ", " << it->second[1] << endl;
            answer.push_back(it->second);
        }
        
        return answer;
    }
};

Suggestions:

• You don't need to bother about whether to store the distance value in double or float. Since you are comparing sqrt(some_value1) with sqrt(some_value2), you can omit the sqrt altogether.
• std::advance has a linear time complexity. You can just simply initialize the iterator at the beginning of the for loop, and increment with ++ operator. This operator has a constant time complexity. Read more about std::advance here .