173. Binary Search Tree Iterator

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

Approach 1: Flattening the BST

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/**
 * 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 {
    ArrayList<Integer> nodeSorted;
    Iterator<Integer> iter;
    public BSTIterator(TreeNode root) {
        this.nodeSorted=new ArrayList<>();
        _inorder(root);
        iter=nodeSorted.iterator();
    }
    
    private void _inorder(TreeNode root){
        if(root==null) return;
        _inorder(root.left);
        nodeSorted.add(root.val);
        _inorder(root.right);
    }
    
    /** @return the next smallest number */
    public int next() {
        if(iter.hasNext()) return iter.next();
        return Integer.MIN_VALUE;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return iter.hasNext();
    }
}

/**
 * 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();
 */

Approach 2: Controlled Recursion using Stack

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/**
 * 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 {
    LinkedList<TreeNode> stack;
    public BSTIterator(TreeNode root) {
        stack=new LinkedList<>();
        _travelLeftMost(root);
    }
    private void _travelLeftMost(TreeNode root){
        while(root!=null){
            stack.push(root);
            root=root.left;
        }
    }
    
    /** @return the next smallest number */
    public int next() {
        TreeNode minNode=stack.pop();
        _travelLeftMost(minNode.right);
        return minNode.val;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return stack.size()>0;
    }
}

/**
 * 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();
 */

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