Binary Search Tree Iterator
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()
andhasNext()
should run in average O(1) time and uses O(h) memory, wherehis the height of the tree.
Solution
Put all left values into stack. Then when getting a next one, pop a node from stack and add the right into stack together with its all left nodes. That will make sure the values are sorted.
public class BSTIterator {
private final Stack<TreeNode> stack;
public BSTIterator(TreeNode root) {
stack = new Stack<>();
TreeNode cur = root;
while (cur != null) {
stack.push(cur);
cur = cur.left;
}
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
return !stack.isEmpty();
}
/** @return the next smallest number */
public int next() {
TreeNode node = stack.pop();
// Traversal cur node's right branch
TreeNode cur = node.right;
while (cur != null){
stack.push(cur);
cur = cur.left;
}
return node.val;
}
}
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