#530二叉搜索树的最小绝对差

import java.util.*;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int val) {
        this.val = val;
    }
}

class Solution {
    public int getMinimumDifference(TreeNode root) {
        TreeNode[] previous = new TreeNode[1];
        int[] answer = {Integer.MAX_VALUE};
        inorder(root, previous, answer);
        return answer[0];
    }

    private void inorder(TreeNode node, TreeNode[] previous, int[] answer) {
        if (node == null) return;
        // BST 中序序列严格递增，最小差只需检查相邻节点
        inorder(node.left, previous, answer);
        if (previous[0] != null) {
            answer[0] = Math.min(answer[0], node.val - previous[0].val);
        }
        // 在进入右子树前同步更新全局中序前驱
        previous[0] = node;
        inorder(node.right, previous, answer);
    }
}