Implement the non-recursive functions nextPreOrder(u), nextInOrder(u), and nextPostOrder(u) that return the node that follows u in a pre-order, in-order, or post-order traversal, respectively. These functions should take amortized constant time; if we start at any node u and repeatedly call one of these functions and assign the return value to u until u = null, then the cost of all these calls should be O(n) in Java please :)