[Leetcode] Binary Search Tree

Title: (Tree Stack)

Implement an iterator over a binary search tree (BST). Your iterator is initialized with the root node of a BST.

Calling would return the next smallest number in the next() BST.

Note: next() hasNext() and should run in average O (1) time and uses O (h) memory, where H is the height of the Tree.

Exercises

Tree's problem is easily linked to stack, the problem is to maintain a stack on it.

With Pushleft ()

/** Definition for binary tree * public class TreeNode {* Int. val; * TreeNode left; * TreeNode right; * TreeNode (int x) {val = x;} }*/ Public classBstiterator {//TreeNode Root;stack<treenode> stack =NewStack<treenode>();  Publicbstiterator (TreeNode root) {pushleft (root); }    /** @return Whether we have a next smallest number*/     PublicBoolean hasnext () {return!Stack.isempty (); }    /** @return The next smallest number*/     Public intNext () {TreeNode small=Stack.pop ();        Pushleft (Small.right); returnSmall.val; }         Public voidpushleft (TreeNode root) { while(root!=NULL) {Stack.push (root); Root=Root.left; }    }}/** * Your bstiterator'll be called like this: * bstiterator i = new Bstiterator (root), * while (I.hasnext ()) v[f ()] = I.next (); */

Constantly push the root.left to the stack, and if that node had right child at the time of the backtracking, then more of that right child would be pushleft ().

[Leetcode] Binary Search Tree