[LeetCode]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() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

题解：使用Stack，由于BST，中序遍历就是从小到大。

code:

public class BSTIterator {

	private Stack<TreeNode> stack = null;

	public BSTIterator(TreeNode root) {
		stack = new Stack<TreeNode>();

		while (root != null) {
			stack.push(root);
			root = root.left;
		}
	}

	/** @return whether we have a next smallest number */
	public boolean hasNext() {
		return !stack.isEmpty();
	}

	/** @return the next smallest number */
	public int next() {
		
		if(hasNext()){
			int ret = stack.peek().val;
			TreeNode cur = stack.pop();
			if(cur.right!=null){
				cur = cur.right;
				while(cur!=null){
					stack.push(cur);
					cur = cur.left;
				}
			}
			return ret;
		}
		return -1;
	}
}