Given a non-empty binary search tree and a target value, find the value in the BST that is closest to the target.
Note:
- Given target value is a floating point.
- You are guaranteed to have only one unique value in the BST that is closest to the target.
思路：递归求解。因为是二叉搜索树，我们不需要遍历所有的节点，通过prune来提高速度。
1 /**
2 * Definition for a binary tree node.
3 * struct TreeNode {
4 * int val;
5 * TreeNode *left;
6 * TreeNode *right;
7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {}
8 * };
9 */
10 class Solution {
11 public:
12 int closestValue(TreeNode* root, double target) {
13 int res = root->val;
14 if (root->left != NULL && (double)res > target) {
15 int leftRes = closestValue(root->left, target);
16 res = abs(res - target) < abs(leftRes - target) ? res : leftRes;
17 }
18 if (root->right != NULL && (double)res < target) {
19 int rightRes = closestValue(root->right, target);
20 res = abs(res - target) < abs(rightRes - target) ? res : rightRes;
21 }
22 return res;
23 }
24 };