Will the Guru

/**
 * 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 {
    private List<Integer> list = new ArrayList<>();
    private int min = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        solve(root);

        for (int i=0;i<list.size()-1;i++) {
            min = Math.min(min, Math.abs(list.get(i) - list.get(i+1)));
        }
        return min;
    }

    private void solve(TreeNode node) {
        if (node == null) {
            return;
        }

        solve(node.left);
        list.add(node.val);
        solve(node.right);
    }
}