leetcode [230]Kth Smallest Element in a BST

佚名 6年前 (2019-05-06) 随笔 232人围观抢沙发百度已收录

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note:
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

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Example 1:

Input: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
Output: 1

Example 2:

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

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

题目大意:

寻找BST中第k小的数字。

解法:

其实就是按中序遍历到的第k个节点。

java:

class Solution {
    int re=0,count=0;
    private void dfs(TreeNode root,int k){
        if(root==null) return;
        dfs(root.left,k);
        count++;
        if(k==count) {
            re=root.val;
            return;
        }
        dfs(root.right,k);
    }

    public int kthSmallest(TreeNode root, int k) {
        dfs(root,k);
        return re;
    }
}