【leetcode hot 100 230】二叉搜索树中第K小的元素

解法一：从小到大输出到list中，取list[k-1]就是第k小的元素

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<Integer> list = new ArrayList<>();

    public int kthSmallest(TreeNode root, int k) {
        // 从小到大输出到list中，取list[k-1]就是第k小的元素
        inoder(root);
        return list.get(k-1); 
    }

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

注意：

• list[k-1]是第k小的元素，而不是list[k]