剑指 Offer 54. 二叉搜索树的第k大节点

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

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

// 打印中序遍历
void dfs(TreeNode root) {
    if(root == null) return;
    dfs(root.left); // 左
    System.out.println(root.val); // 根
    dfs(root.right); // 右
}

// 打印中序遍历倒序
void dfs(TreeNode root) {
    if(root == null) return;
    dfs(root.right); // 右
    System.out.println(root.val); // 根
    dfs(root.left); // 左
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private ArrayList<Integer> res = new ArrayList<Integer>(); 
    public int kthLargest(TreeNode root, int k) {

        inOrder(root);
        int result = res.get(k-1);
        return result;
    }

    public void inOrder(TreeNode root)
    {
        if(root == null) return;
        inOrder(root.right);
        res.add(root.val);
        inOrder(root.left);
    }
}

class Solution {
    int res, k;
    public int kthLargest(TreeNode root, int k) {
        this.k = k;
        dfs(root);
        return res;
    }
    void dfs(TreeNode root) {
        if(root == null) return;
        dfs(root.right);
        if(k == 0) return;
        if(--k == 0) res = root.val;
        dfs(root.left);
    }
}