173. Binary Search Tree Iterator

Medium (LinkedIn, Google, Facebook, Microsoft)

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

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

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

/**

• Definition for binary tree
• public class TreeNode {
• int val;
• TreeNode left;
• TreeNode right;
• TreeNode(int x) { val = x; }
• } */

public class BSTIterator {

private Stack<TreeNode> stack;
private TreeNode root;

public BSTIterator(TreeNode root) {
    stack = new Stack<TreeNode>();
    this.root = root;
}

/** @return whether we have a next smallest number */
public boolean hasNext() {
    return !stack.isEmpty() || root != null;
}

/** @return the next smallest number */
public int next() {
    while(root != null) {
        stack.push(root);
        root = root.left;
    }

    root = stack.pop();
    int ans = root.val;
    root = root.right;
    return ans;
}

}