Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int ans;
    private double target;
    private double diff = Double.MAX_VALUE;

    public int closestValue(TreeNode root, double target) {
        this.target = target;
        dfs(root);
        return ans;
    }

    private void dfs(TreeNode node) {
        if (node == null) {
            return;
        }
        double nxt = Math.abs(node.val - target);
        if (nxt < diff || (nxt == diff && node.val < ans)) {
            diff = nxt;
            ans = node.val;
        }
        node = target < node.val ? node.left : node.right;
        dfs(node);
    }
}