leetcode-530. Minimum Absolute Difference in BST

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).

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

题意：找出二叉搜索树中，结点差值最小的差值。
解法：因为要找出结点差值最小的值，本来是要遍历N*N才能找到的，但是因为是二叉搜索树，中序遍历的结构就是有序的，最小差值肯定是相邻结点的差值中的一个，所以只要O(n)就可以了。

public class Solution {
int min = Integer.MAX_VALUE;
Integer 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);
}
prev = root.val;

getMinimumDifference(root.right);

return min;
}

}