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import java.util.Stack;

public class BSTIterator {

private Stack<TreeNode> mStack;

private TreeNode mCurNode;

public BSTIterator(TreeNode root) {

mStack = new Stack<>();

mCurNode = root;

}

/**

* @return whether we have a next smallest number

*/

public boolean hasNext() {

return !mStack.isEmpty() || mCurNode != null;

}

/**

* @return the next smallest number

*/

public int next() {

int result = -1;

while (hasNext()) {

if (mCurNode != null) {

mStack.push(mCurNode);

mCurNode = mCurNode.left;

} else {

mCurNode = mStack.pop();

result = mCurNode.val;

mCurNode = mCurNode.right;

break;

}

}

return result;

}

}