173_Binary_search_tree_iterator

// https://leetcode.com/problems/binary-search-tree-iterator/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class BSTIterator {
    List<TreeNode> list = new ArrayList();
    int count = 0;
    int size = 0;
    public BSTIterator(TreeNode root) {
        Stack<TreeNode> stack = new Stack();
        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            list.add(root);
            root = root.right;
        }
        size = list.size()-1;
        System.out.println(size);
    }
    
    /** @return the next smallest number */
    public int next() {
        if (count <= size) {
            TreeNode node = list.get(count++);
            if (node == null)
                return 0;
            else
                return node.val;
        }
        return -1;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        if (count <= size)
            return true;
        else
            return false;
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */
 
 // BEST APPROACH
 
 /**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */
class BSTIterator {

    Stack<TreeNode> stack;

    public BSTIterator(TreeNode root) {
        
        // Stack for the recursion simulation
        this.stack = new Stack<TreeNode>();
        
        // Remember that the algorithm starts with a call to the helper function
        // with the root node as the input
        this._leftmostInorder(root);
    }

    private void _leftmostInorder(TreeNode root) {
      
        // For a given node, add all the elements in the leftmost branch of the tree
        // under it to the stack.
        while (root != null) {
            this.stack.push(root);
            root = root.left;
        }
    }

    /**
     * @return the next smallest number
     */
    public int next() {
        // Node at the top of the stack is the next smallest element
        TreeNode topmostNode = this.stack.pop();

        // Need to maintain the invariant. If the node has a right child, call the 
        // helper function for the right child
        if (topmostNode.right != null) {
            this._leftmostInorder(topmostNode.right);
        }

        return topmostNode.val;
    }

    /**
     * @return whether we have a next smallest number
     */
    public boolean hasNext() {
        return this.stack.size() > 0;
    }
}