900. Closest Binary Search Tree Value

描述

Given a non-empty binary search tree and a target value, find the value in the BST that is closest to the target.

• 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.

您在真实的面试中是否遇到过这个题？  是

样例

Given root = {1}, target = 4.428571, return 1.

我的第一想法就是先序遍历即可，但是这样做遗漏了最重要的一个条件这是一个二叉搜索树，如果目标比当前节点的值要大，那就没有必要遍历当前节点的左子树了。所以遍历的时候加上筛选条件即可。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: the given BST
     * @param target: the given target
     * @return: the value in the BST that is closest to the target
     */
    int closestValue(TreeNode * root, double target) {
        // write your code here
        int a=root->val;
        TreeNode *t=target<a?root->left:root->right;
        if(!t) return a;
        int b=closestValue(t,target);
        return abs(a-target)<abs(b-target)?a:b;
    }
};