题解 | #二叉搜索树的第k个节点#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param proot TreeNode类
     * @param k int整型
     * @return int整型
     */
    public int KthNode (TreeNode proot, int k) {
        // write code here

        // 一些特殊情况的处理
        if (null == proot || 0 == k) {
            return -1;
        }

        Stack<TreeNode> stack = new Stack<>();
        TreeNode tmpNode = proot;
        int p = 0; // 记录当前是第几个节点
        int rs = Integer.MAX_VALUE;

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

        while (!stack.isEmpty()) {
            tmpNode = stack.pop();
            p++;
            if (p == k) {
                rs = tmpNode.val;
                break;
            }
            if (null != tmpNode.right) {
                tmpNode = tmpNode.right;
                while (null != tmpNode) {
                    stack.push(tmpNode);
                    tmpNode = tmpNode.left;
                }
            }
        }
        return rs == Integer.MAX_VALUE ? -1 : rs;
    }
}