530，二叉搜索树的最小绝对差

给你一个二叉搜索树的根节点 root ，返回 树中任意两不同节点值之间的最小差值 。

差值是一个正数，其数值等于两值之差的绝对值。

双指针省空间 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int minValue = Integer.MAX_VALUE;
    TreeNode pre = null;
    void travel(TreeNode root){
        if(root==null) return;
        travel(root.left);
        if(pre!=null){
            int temp  = Math.abs(root.val-pre.val);
            minValue = minValue<temp?minValue:temp;
        }
        pre = root;//双指针
        travel(root.right);
    }
    public int getMinimumDifference(TreeNode root) {
        travel(root);
        return minValue;
    }
}