It looks like you are conflating a linked list with a heap. Once you have an array/vector for your node list the unused memory can be a list itself, each index

It looks like you are conflating a linked list with a heap. Once you have an array/vector for your node list the unused memory can be a list itself, each index pointing to the next free index. You just need the next free index to get memory and you point to the current free index when you free a node. You don't need to manage the memory further than that.