Leetcode: Closest Binary Search Tree Value II
Given a non-empty binary search tree and a target value, find k values in the BST that are closest to the target.
Note:
Given target value is a floating point.
You may assume k is always valid, that is: k ≤ total nodes.
You are guaranteed to have only one unique set of k values in the BST that are closest to the target.
Follow up:
Assume that the BST is balanced, could you solve it in less than O(n) runtime (where n = total nodes)?
Hint:
- Consider implement these two helper functions:
getPredecessor(N)
, which returns the next smaller node to N.getSuccessor(N)
, which returns the next larger node to N.
- Try to assume that each node has a parent pointer, it makes the problem much easier.
- Without parent pointer we just need to keep track of the path from the root to the current node using a stack.
- You would need two stacks to track the path in finding predecessor and successor node separately.
用一个栈一个队列,用recursion写inorder Traversal, Time: O(N+K), Space: O(N)
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 public class Solution { 11 public List<Integer> closestKValues(TreeNode root, double target, int k) { 12 List<Integer> res = new ArrayList<Integer>(); 13 LinkedList<Integer> stack = new LinkedList<Integer>(); 14 LinkedList<Integer> queue = new LinkedList<Integer>(); 15 inorder(root, target, stack, queue); 16 for (int i=0; i<k; i++) { 17 if (stack.isEmpty()) res.add(queue.poll()); 18 else if (queue.isEmpty()) res.add(stack.pop()); 19 else { 20 int s = stack.peek(); 21 int q = queue.peek(); 22 if (Math.abs(s-target) < Math.abs(q-target)) { 23 res.add(s); 24 stack.pop(); 25 } 26 else { 27 res.add(q); 28 queue.poll(); 29 } 30 } 31 } 32 return res; 33 } 34 35 public void inorder(TreeNode root, double target, LinkedList<Integer> stack, LinkedList<Integer> queue) { 36 if (root == null) return; 37 inorder(root.left, target, stack, queue); 38 if (root.val < target) { 39 stack.push(root.val); 40 } 41 else { 42 queue.offer(root.val); 43 } 44 inorder(root.right, target, stack, queue); 45 } 46 }
Better Idea: 只用一个大小为K的队列的方法:Time O(N), Space O(K)
前K个数直接加入队列,之后每来一个新的数(较大的数),如果该数和目标的差,相比于队头的数离目标的差来说,更小,则将队头拿出来,将新数加入队列。如果该数的差更大,则直接退出并返回这个队列,因为后面的数更大,差值也只会更大。
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 public class Solution { 11 public List<Integer> closestKValues(TreeNode root, double target, int k) { 12 //List<Integer> res = new ArrayList<Integer>(); 13 LinkedList<TreeNode> stack = new LinkedList<TreeNode>(); 14 LinkedList<Integer> queue = new LinkedList<Integer>(); 15 16 while (!stack.isEmpty() || root!=null) { 17 if (root != null) { 18 stack.push(root); 19 root = root.left; 20 } 21 else { 22 root = stack.pop(); 23 if (queue.size() < k) { 24 queue.offer(root.val); 25 } 26 else { 27 if (Math.abs(root.val-target) < Math.abs(queue.peek()-target)) { 28 queue.poll(); 29 queue.offer(root.val); 30 } 31 else break; 32 } 33 root = root.right; 34 } 35 } 36 37 return (List<Integer>)queue; 38 } 39 40 }