530. 二叉搜索树的最小绝对差

解法一：中序遍历

对二叉搜索树进行中序遍历可以得到升序序列，最小绝对差可以从相邻两个结点的差中取得。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int minDelta;
    private TreeNode last;
    public int getMinimumDifference(TreeNode root) {
        minDelta = Integer.MAX_VALUE;
        last = null;
        inOrder(root);
        return minDelta;
    }
    private void inOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        inOrder(root.left);
        if (last != null) {
            minDelta = Math.min(minDelta, root.val - last.val);
        }
        last = root;
        inOrder(root.right);
    }
}