I'm working on binary search trees for CS class, does anyone know a good way of recursively putting all the nodes into a list? The way i'm doing it right now works, but it produces a list with nested lists inside it. However, given the operations i can use on the list, i'm not sure if i can flatten it.

/* For the key values 1...numKeys, how many structurally unique binary search trees are possible that store those keys. Strategy: consider that each value could be the root. Recursively find the size of the left and right subtrees. */ int countTrees(int numKeys) { if (numKeys <=1) { return(1); } else { // there will be one value at the root, with whatever remains // on the left and right each forming their own subtrees. // Iterate through all the values that could be the root... int sum = 0; int left, right, root; for (root=1; root<=numKeys; root++) { left = countTrees(root - 1); right = countTrees(numKeys - root); // number of possible trees with this root == left*right sum += left*right; } return(sum); } }