173-二叉搜索树迭代器.java

import java.util.ArrayDeque;
import java.util.Deque;

// Definition for a binary tree node.
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

class BSTIterator {
    private final Deque<TreeNode> stack = new ArrayDeque<>();

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

    /**
     * @return the next smallest number
     */
    public int next() {
        var res = stack.pop();
        var cur = res.right;
        while (cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
        return res.val;
    }

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

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */