Closest Binary Search

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.

Analyse: get the value of each node. Since we know that the closest node is either the root node, or its child node. Use the property of binary search tree, find the value of the corresponding subtree. Compare the diff between root & target and child & target. 

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int closestValue(TreeNode* root, double target) {
        TreeNode* child = target < root->val ? root->left : root->right;
        if (!child) return root->val;
        int childValue = closestValue(child, target);
        return abs(root->val - target) < abs(childValue - target) ? root->val : childValue;
    }
};