src/main/java/com/fishercoder/solutions/_272.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class _272 {

    public static class Solution1 {
        public List<Integer> closestKValues(TreeNode root, double target, int k) {
            List<Integer> res = new ArrayList();

            Stack<Integer> s1 = new Stack(); // predecessors
            Stack<Integer> s2 = new Stack(); // successors

            inorder(root, target, false, s1);
            inorder(root, target, true, s2);

            while (k-- > 0) {
                if (s1.isEmpty()) {
                    res.add(s2.pop());
                } else if (s2.isEmpty()) {
                    res.add(s1.pop());
                } else if (Math.abs(s1.peek() - target) < Math.abs(s2.peek() - target)) {
                    res.add(s1.pop());
                } else {
                    res.add(s2.pop());
                }
            }

            return res;
        }

        // inorder traversal
        void inorder(TreeNode root, double target, boolean reverse, Stack<Integer> stack) {
            if (root == null) {
                return;
            }

            inorder(reverse ? root.right : root.left, target, reverse, stack);
            // early terminate, no need to traverse the whole tree
            if ((reverse && root.val <= target) || (!reverse && root.val > target)) {
                return;
            }
            // track the value of current node
            stack.push(root.val);
            inorder(reverse ? root.left : root.right, target, reverse, stack);
        }
    }
}