二叉搜索树的第k个结点

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);
  }
}