[Solved] why does this code gives runtime error?

by


a is declared a an empty vector so you cannot access elements of a unless there are any elements in a.
You want 

  a.push_back(nums[i]);

instead of

  a[k] = nums1[i]; //a is declared as empty vector so you have to push elements to it.

OR

you can do 

vector<int> a(num1.size());

instead of

vector<int> a;

solved why does this code gives runtime error?