/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
* Example of iterate a tree:
* BSTIterator iterator = new BSTIterator(root);
* while (iterator.hasNext()) {
* TreeNode node = iterator.next();
* do something for node
* }
*/
public class BSTIterator {
private Deque<TreeNode> stack;
private TreeNode next;
/**
* @param root: The root of binary tree.
*/
public BSTIterator(TreeNode root) {
stack = new ArrayDeque<TreeNode>();
next = root;
}
/**
* @return: True if there has next node, or false
*/
public boolean hasNext() {
if (next != null) {
TreeNode curr = next;
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
next = null;
}
return !stack.isEmpty();
}
/**
* @return: return next node
*/
public TreeNode next() {
if (!hasNext())
return null;
TreeNode curr = stack.pop();
next = curr.right;
return curr;
}
}
Notes
Use stack to store all the left children.
If the stack is empty, then there is no next node.
The top of the stack is the current node. In terms of the tree structure, the next node of the current node is its right child.
