Using Multiset

[Pen72]

The way to swing the list follows the algorithm below :

1Pick the smallest integer from s and append it to the result.
2Pick the smallest integer from which is greater than the last appended integer to the result and append it.
3Repeat step 2 until you cannot pick more integers.
4Pick the largest integer from s and append it to the result.
5Pick the largest integer from s which is smaller than the last appended integer to the result and append it.
6Repeat step 5 until you cannot pick more integer.
7Repeat the steps from 1 to 6 until you pick all integers from s .


Input Format

The first line has a integer meaning there are n lists.

The next n line, each line has a list. The first integer l of a line is the length of the list. And followed integers are the elements of the list.

The code below shows segmentation fault, and I'm thinking the way I use the multiset iterator isn't applicable.

#include <cmath>
#include <cstdio>
#include <vector>
#include <queue>
#include <list>
#include <iostream>
#include <algorithm>
#include <set>
#include <iterator>
using namespace std;


int main() {
   int n;
    cin>>n;
    while(n--)
    {
       int L,val;
        cin>>L;  
        multiset<int> A;
        A.clear();
        for(int i=0;i<L;i++)
        { int x; cin>>x;  A.insert(x);}
        
        list<int> myList = list<int>();
        while(!A.empty())
        {
            val=*(A.begin());
            myList.push_back(val);
            A.erase(A.begin());
        
            while(*(A.begin())>val)
            {
             val=*(A.begin());
             myList.push_back(val);
             A.erase(A.begin());
            }

            val=*(A.end());
            myList.push_back(val);
            A.erase(A.end());
            
             while(*(A.end())<val)
            {
             val=*(A.end());
             myList.push_back(val);
             A.erase(A.end());
            }
            
        }

        for(auto v: myList)
        cout<<v<<" ";
        cout<<"\n";
        
    }
    return 0;
}

[seeplus]

and it faults at which line? Where does tracing through the code using the debugger cause an issue?

For run-time issues, use the debugger to find where is the problem.

[seeplus]

L35. After the .erase() at L39, the iterator it is invalid as the entry has been removed. Trying to de-reference an invalid iterator is a no-no .erase() returns a valid iterator so don't you mean something like:

while((*it)>val) {
    val=*it;
    myList.push_back(val);
    it = A.erase(it);
}

and similarly for elsewhere in the code. When using iterators, you always need to make sure they are valid.

[seeplus]

[new thread at http://www.cplusplus.com/forum/beginner/282916/ ]