#yyds干货盘点# LeetCode程序员面试金典：二叉搜索树中第K小的元素

输入服务器托管网：root = [3,1,4,null,2], k = 1
输出：1

输入：root = [5,3,6,2,4,null,null,1], k = 3
输出：3

class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Deque stack = new ArrayDeque();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            --k;
            if (k == 0) {
                break;
            }
            root = root.right;
        }
        return root.val;
    }
}