二叉搜索树的第k个结点

Posted 2021-01-12 MarkLeeBYR

题目描述

给定一颗二叉搜索树，请找出其中的第k大的结点。例如， 5 / \ 3 7 /\ /\ 2 4 6 8 中，按结点数值大小顺序第三个结点的值为4。

此树按照中序遍历打出来是[2 3 4 5 6 7 8]，所以第三大的节点是4

Solution 1://借助stack

public class Solution {

    int count = 0;

    TreeNode KthNode(TreeNode pRoot, int k) {

        if(count > k || pRoot == null)

            return null;

        TreeNode p = pRoot;

        Stack<TreeNode> stack = new Stack<>();

        TreeNode kthNode = null;

        stack.push(pRoot);

        while(!stack.isEmpty()){

            TreeNode tn = stack.peek();

            while(tn != null){

                tn = tn.left;

                stack.push(tn);

            }

            stack.pop();

            if (!stack.isEmpty()) {

                tn = stack.pop();

                count++;

                if (count == k)

                    kthNode = tn;

                stack.push(tn.right);

            }

        }

        return kthNode; 

    }  

}

Solution 2:

public class Solution {

    int k;

    TreeNode temp = null;

    public TreeNode KthNode(TreeNode pRoot, int k) {

              this.k = k;

              helper(pRoot);

              return temp;

    }

 

    public void helper(TreeNode pRoot) {

             if (pRoot == null)

                 return;

             helper(pRoot.left);

             if (--k == 0)

                 temp = pRoot;

             helper(pRoot.right);

    }

}