173. 二叉搜索树迭代器

/**
 * Definition for binary tree
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    Stack<TreeNode> st = new Stack<TreeNode>();


    public BSTIterator(TreeNode root) {
        while (root != null) {
            st.push(root);
            root = root.left;
        }
    }

    /**
     * @return whether we have a next smallest number
     */
    public boolean hasNext() {
        if (!st.isEmpty()) {
            return true;
        } else {
            return false;
        }
    }

    /**
     * @return the next smallest number
     */
    public int next() {
        TreeNode curr = st.pop();
        int val = curr.val;
        if (curr.right != null) {
            curr = curr.right;
            st.push(curr);
            // Push the left child of curr.right into stack
            while (curr.left != null) {
                st.push(curr.left);
                curr = curr.left;
            }
        }
        return val;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */