# [LeetCode] 530. Minimum Absolute Difference in BST

## Problem

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).

Note: There are at least two nodes in this BST.

## Iterative -- using stack

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

## recursive -- in-order

``````class Solution {
Integer pre = null;
int min = Integer.MAX_VALUE;
public int getMinimumDifference(TreeNode root) {
if (root == null) return min;
getMinimumDifference(root.left);

if (pre != null) {
min = Math.min(min, root.val-pre);
}
pre = root.val;

getMinimumDifference(root.right);

return min;
}
}``````