# Minimum Absolute Difference in BST

## Question

### Problem Statement

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.

## 题解

### Java - Recursive

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
private int min = Integer.MAX_VALUE;
private TreeNode prev = null;

public int getMinimumDifference(TreeNode root) {
if (root == null) return min;

getMinimumDifference(root.left);
if (prev != null) {
min = Math.min(min, root.val - prev.val);
}
prev = root;
getMinimumDifference(root.right);
return min;
}
}


### Java - Iterative

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int getMinimumDifference(TreeNode root) {
int min = Integer.MAX_VALUE;
TreeNode prev = null;
Deque<TreeNode> stack = new ArrayDeque<TreeNode>();

while (root != null || (!stack.isEmpty())) {
if (root != null) {
stack.push(root);
root = root.left;
} else {
root = stack.pop();
if (prev != null) {
min = Math.min(min, root.val - prev.val);
}
prev = root;
root = root.right;
}
}

return min;
}
}