Java描述 LeetCode，剑指 Offer 54. 二叉搜索树的第k大节点

𝑰’𝒎 𝒉𝒉𝒈, 𝑰 𝒂𝒎 𝒂 𝒈𝒓𝒂𝒅𝒖𝒂𝒕𝒆 𝒔𝒕𝒖𝒅𝒆𝒏𝒕 𝒇𝒓𝒐𝒎 𝑵𝒂𝒏𝒋𝒊𝒏𝒈, 𝑪𝒉𝒊𝒏𝒂.

• 🏫 𝑺𝒉𝒄𝒐𝒐𝒍: 𝑯𝒐𝒉𝒂𝒊 𝑼𝒏𝒊𝒗𝒆𝒓𝒔𝒊𝒕𝒚
• 🌱 𝑳𝒆𝒂𝒓𝒏𝒊𝒏𝒈: 𝑰’𝒎 𝒄𝒖𝒓𝒓𝒆𝒏𝒕𝒍𝒚 𝒍𝒆𝒂𝒓𝒏𝒊𝒏𝒈 𝒅𝒆𝒔𝒊𝒈𝒏 𝒑𝒂𝒕𝒕𝒆𝒓𝒏, 𝑳𝒆𝒆𝒕𝒄𝒐𝒅𝒆, 𝒅𝒊𝒔𝒕𝒓𝒊𝒃𝒖𝒕𝒆𝒅 𝒔𝒚𝒔𝒕𝒆𝒎, 𝒎𝒊𝒅𝒅𝒍𝒆𝒘𝒂𝒓𝒆 𝒂𝒏𝒅 𝒔𝒐 𝒐𝒏.
• 💓 𝑯𝒐𝒘 𝒕𝒐 𝒓𝒆𝒂𝒄𝒉 𝒎𝒆：𝑽𝑿
• 📚 𝑴𝒚 𝒃𝒍𝒐𝒈: 𝒉𝒕𝒕𝒑𝒔://𝒉𝒉𝒈𝒚𝒚𝒅𝒔.𝒃𝒍𝒐𝒈.𝒄𝒔𝒅𝒏.𝒏𝒆𝒕/
• 💼 𝑷𝒓𝒐𝒇𝒆𝒔𝒔𝒊𝒐𝒏𝒂𝒍 𝒔𝒌𝒊𝒍𝒍𝒔：𝒎𝒚 𝒅𝒓𝒆𝒂𝒎
  

1-1: Description

给定一棵二叉搜索树，请找出其中第 k 大的节点的值。

示例 1:

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

示例 2:

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

限制：

1 ≤ k ≤ 二叉搜索树元素个数

1-2: Inverse Inorder Traverse

As we know, the result of inorder traversal of a binary tree is increasing. At the same time, the inverse inorder traversal result is decreasing. We can use this feature to resolve the question. Here is my code:

int result, k;

public int kthLargest(TreeNode root, int k) {
    this.k = k;
    dfs(root);
    return result;
}

public void dfs(TreeNode root) {
    if (root == null || k <= 0) {
        return;
    }
    dfs(root.right);
    // 已经找到对应的k了，剪枝
//        if (k == 0) {
//            return;
//        }
    k--;
    if (k == 0) {
        result = root.val;
        return;
    }
    dfs(root.left);

Notes: As the recursive function running, they all use the same value k. So, we use the member variable k in the object;