Question

I am using a stl map on gcc comper which uses a tree to store key,value pairs.
The iterator advances in an inorder fashion so inorder traversal is really easy.
However one of my output requirements is a postorder traversal.
I have been specifically aksed to use map.
Is there any way to get it done?

Answer 1

As Steve Jessop said Map is the abstract data structure provided by c++, you can not change the order of elements stored in map in any order, like we can do in Tree or AVL by traversing in pre/post/in order.
but you can reverse the order of storing by using std::greater as your key instead of std::less.

e.g.

std::map< std::string, int, std::greater<std::string> > my_map; 

OR
crate one Vector desiredOrder, which will keep the trac of order of insertion in map, and once you are done with insertion just do whatever sorting operation you want pre/post/ in order on the desiredOrder Vector and using for loop you can traverse the vector and access the Map elements using the vector value as a key for your Map

e.g

std::vector<std::string> desiredOrder;
std::map<std::string, int> myTree;

myTree["A"] = 0;
desiredOrder.push_back("A"); 
myTree["B"] = 0;
desiredOrder.push_back("B");
myTree["C"] = 0;
desiredOrder.push_back("C");
myTree["D"] = 0;
desiredOrder.push_back("D");

/*
Do whatever sorting you want to do here....
*/

for (int i = 0; i < desiredOrder.size(); ++i)
{
    const std::string &s = desiredOrder[i];
    std::cout << s << ' ' << myTree[s] << '\n';
}