1ms Java recursive solution


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

  • 0

    Below is 0ms, O(1) space solution (however it has global variables)

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        private int val = 0;
        private int curr = 0;
        public int kthSmallest(TreeNode root, int k) {
            fillList(root, k);
            return val;
        }
        private void fillList(TreeNode root, int k) {
            if(root == null) return;
            else if(curr < k){
                fillList(root.left, k);
                curr++;
                if(curr == k) val = root.val;
                fillList(root.right, k);
            }
            return;
        }
    }
    

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