530. Minimum Absolute Difference in BST

Given a binary search tree with non-negative values, find the minimum absolute difference between values of any two nodes.

Example:

Input:

   1
    
     3
    /
   2

Output:
1

Explanation:
The minimum absolute difference is 1, which is the difference between 2 and 1 (or between 2 and 3).

class Solution {
    int min = Integer.MAX_VALUE;
    TreeNode pre = null;
    public int getMinimumDifference(TreeNode root) {
        if(root == null) return min;
        
        getMinimumDifference(root.left);
        if(pre != null) min = Math.min(min, root.val - pre.val);
        pre = root;
        getMinimumDifference(root.right);
        return min;
    }
}

BST inorder遍历是从小到大的，所以我们比较相邻两个的diff就能得到min，pre可以设成数字或者node都可以