leetcode 230. Kth Smallest Element in a BST

Seawolf

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.

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?

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return 0;
        help(root,list);
        Integer[] re = new Integer[list.size()];
        list.toArray(re);
        Arrays.sort(re);
        
        return re[k-1];
    }
    public void help(TreeNode root, List<Integer> list){
        if(root == null) return;
        if(root != null){
            help(root.left,list);
            list.add(root.val);
            help(root.right,list);
        }
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return 0;
        help(root,list);
        Integer[] re = new Integer[list.size()+1];
        
        for(int i = 0; i < list.size(); i++){
            re[list.get(i)] = 0;
            re[list.get(i)]++;
        }
        for(int i = 0; i< re.length; i++){
            if(re[i] == null) continue;
            if(re[i] == 1) k--;
            if(k == 0) return i;
        }
        return 0;
    }
    public void help(TreeNode root, List<Integer> list){
        if(root == null) return;
        if(root != null){
            help(root.left,list);
            list.add(root.val);
            help(root.right,list);
        }
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return 0;
        help(root,list);
        
        return list.get(k-1);
    }
    public void help(TreeNode root, List<Integer> list){
        if(root == null) return;
        if(root != null){
            help(root.left,list);
            list.add(root.val);
            help(root.right,list);
        }
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int kth = 0;
    int result = 0;
    public int kthSmallest(TreeNode root, int k) {
        kth = k;
        help(root);
        return result;
        
    }
    private void help(TreeNode root){
        if(root == null || kth == 0) return;
        
        help(root.left);
        kth--;
        if(kth == 0) result = root.val;
        help(root.right);
    }
}