解题思路：

中序遍历，与前一个节点的值进行比较

class Solution {Integer min = Integer.MAX_VALUE;TreeNode pre = null;public int getMinimumDifference(TreeNode root) {traversal(root);return min;}public void traversal(TreeNode cur) {if (cur == null) return;traversal(cur.left);if (pre != null) {if (min > cur.val - pre.val)min = cur.val - pre.val;}pre = cur;traversal(cur.right);}
}