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).

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题目要求找到二叉搜索树中，节点之间最小的差值。我们知道二叉搜索树的中序遍历为有序的，最小的差值就出现在相邻元素之间的差值。

class Solution {

    int min = Integer.MAX_VALUE;

    Integer prev = null; //记录前一个节点，可以定义为TreeNode

    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;

    }

}