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.
思路：
代码：
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/ public class BSTIterator { private TreeNode cur;
private Stack<TreeNode> stack; public BSTIterator(TreeNode root) {
cur = root;
stack = new Stack<>(); } /** @return whether we have a next smallest number */
public boolean hasNext() {
if(!stack.isEmpty()|| cur!=null) return true;
return false;
} /** @return the next smallest number */
public int next() {
while(cur!=null){
stack.push(cur);
cur = cur.left;
}
cur = stack.pop();
int val = cur.val;
cur = cur.right;
return val;
}
} /**
* Your BSTIterator will be called like this:
* BSTIterator i = new BSTIterator(root);
* while (i.hasNext()) v[f()] = i.next();
*/