Line 1034: Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h) in leetcode IDE

Question

I have tried a problem in leetcode IDE. Got a runtime error on running the below code.

As I am a beginner, I'm not able to debug the error.

class Solution 
    {
       public:
             vector<int> runningSum(vector<int>& nums) {
             vector<int> result;
             int  n=nums.size();
             for(int i=0;i<n;i++)
             {
                int sum=0;
                for(int j=0;j<=i;j++)
                {
                   sum=sum + nums[j];
                }
                result[i]=sum;
                
            }
            return result;
        }
    };

Answer 1

The problem in your solution is that result[i] is not a defined address because result vector is empty, it does not have any size defined.

You can either use result.push_back(sum); or while declaring vector<int> result(n); but not both.

Complete code

class Solution 
    {
       public:
       vector<int> runningSum(vector<int>& nums) {
             int  n=nums.size();
             vector<int> result(n);
             
             for(int i=0;i<n;i++)
             {
                int sum=0;
                for(int j=0;j<=i;j++)
                {
                   sum=sum + nums[j];
                }
                result[i]=sum;
                
            }
            return result;
        }
    };

Unrelated but there is an efficient and shorter way to implement this code. Your code has a time complexity of O(n²) but it can be done in O(n) like below:

class Solution 
    {
       public:
       vector<int> runningSum(vector<int>& nums) {
             int  n=nums.size();
             vector<int> result(n);
             
             result[0] = nums[0];
             for(int i=1;i<n;i++)
             {
                result[i] = result[i-1] + nums[i];
             }
             return result;
        }
    };