# Minimum Absolute Difference in BST

Given a binary search tree with non-negative values, find the minimum [absolute difference](https://en.wikipedia.org/wiki/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).
```

## Note

中序遍历

## Code

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

        return res;
    }
}
```

```java
class Solution {
    public int getMinimumDifference(TreeNode root) {
        int res = Integer.MAX_VALUE;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        TreeNode pre = null; 
        while (cur != null || !stack.isEmpty()) {
            while (cur != null) {
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            if (pre != null) {
                res = Math.min(res, cur.val - pre.val);
            }
            pre = cur;
            cur = cur.right;
        }

        return res;
    }
}
```
