530. 二叉搜索树的最小绝对差

给定一个所有节点为非负值的二叉搜索树，求树中任意两节点的差的绝对值的最小值。

示例 :

输入:

1
\
3
/
2

输出:
1

解释:
最小绝对差为1，其中 2 和 1 的差的绝对值为 1（或者 2 和 3）。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/minimum-absolute-difference-in-bst

public class MinimumAbsoluteDiffernceInBST530 {
    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) {
            val = x;
        }
    }
    
/*    private int minDiff = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        TreeNode preNode = null;
        inOrder(root, preNode);
        return minDiff;
    }
    public void inOrder(TreeNode root, TreeNode preNode) {
        if(root == null) {
            return;
        }
        inOrder(root.left, preNode);
        if(preNode != null) {
            minDiff = Math.min(minDiff, root.val - preNode.val);
        }
        preNode = root;
        inOrder(root.right, preNode);        
    }
    
    public static void main(String[] args) {
        MinimumAbsoluteDiffernceInBST530 test = new MinimumAbsoluteDiffernceInBST530();
        TreeNode root = new TreeNode(5);
        root.left = new TreeNode(4);
        root.right = new TreeNode(7);
        System.out.println(test.getMinimumDifference(root));
    }*/
    private int minDiff = Integer.MAX_VALUE;
    private TreeNode preNode = null; 
    public int getMinimumDifference(TreeNode root) {      
        inOrder(root);
        return minDiff;
    }
    public void inOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        inOrder(root.left);
        if(preNode != null) {
            minDiff = Math.min(minDiff, root.val - preNode.val);
        }
        preNode = root;
        inOrder(root.right);        
    }
    
    public static void main(String[] args) {
        MinimumAbsoluteDiffernceInBST530 test = new MinimumAbsoluteDiffernceInBST530();
        TreeNode root = new TreeNode(5);
        root.left = new TreeNode(4);
        root.right = new TreeNode(7);
        System.out.println(test.getMinimumDifference(root));
    }
}