270. Closest Binary Search Tree Value 二叉搜索树中，距离目标值最近的节点
Given the root of a binary search tree and a target value, return the value in the BST that is closest to the target.
Example 1:
Input: root = [4,2,5,1,3], target = 3.714286
Output: 4
Example 2:
Input: root = [1], target = 4.428571
Output: 1
复习时还不会的地方：我缺少了一个比较的过程。这种有大小的题是需要比大小的。node已经很大了就去左边寻找，
……小……右边
参考：https://leetcode.com/problems/closest-binary-search-tree-value/discuss/70322/Super-clean-recursive-Java-solution
public class Solution { public int closestValue(TreeNode root, double target) { return closest(root, target, root.val); } private int closest(TreeNode node, double target, int val) { if (node == null) return val; if (Math.abs(node.val - target) < Math.abs(val - target)) val = node.val; if (node.val < target) val = closest(node.right, target, val); else if (node.val > target) val = closest(node.left, target, val); return val; } }
public class Solution { public int closestValue(TreeNode root, double target) { return closest(root, target, root.val); } private int closest(TreeNode node, double target, int val) { if (node == null) return val; if (Math.abs(node.val - target) < Math.abs(val - target)) val = node.val; if (node.val < target) val = closest(node.right, target, val); else if (node.val > target) val = closest(node.left, target, val); return val; } }