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BSTIterator.java
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74 lines (60 loc) · 1.66 KB
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package leetcode;
import java.util.Stack;
/* O(1) time and uses O(h) memory */
public class BSTIterator {
Stack<TreeNode> stack;
public BSTIterator(TreeNode root) {
stack = new Stack<TreeNode>();
helper(root);
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
return !stack.isEmpty();
}
/** @return the next smallest number */
public int next() {
TreeNode next = stack.pop();
helper(next.right);
return next.val;
}
public void helper(TreeNode root){
while(root != null){
stack.push(root);
root = root.left;
}
}
public static void main(String[] args) {
TreeNode root = new TreeNode(4);
root.left = new TreeNode(2);
root.right = new TreeNode(5);
root.left.left = new TreeNode(1);
root.left.right = new TreeNode(3);
root.right.right = new TreeNode(6);
BSTIterator bsti = new BSTIterator(root);
while(bsti.hasNext()){
System.out.println(bsti.next());
}
}
}
class BSTIterator1 {
/* not O(h) memory*/
Stack<TreeNode> stack;
public BSTIterator1(TreeNode root) {
stack = new Stack<TreeNode>();
inorder(stack, root);
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
return !stack.isEmpty();
}
/** @return the next smallest number */
public int next() {
return stack.pop().val;
}
public void inorder(Stack<TreeNode> stack, TreeNode root){
if(root == null) return;
if(root.right != null) inorder(stack, root.right);
stack.push(root);
if(root.left != null) inorder(stack, root.left);
}
}