アプローチ

DFS

/**
 * 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 {

    public int result = Integer.MAX_VALUE;
    public ArrayList<Integer> list;
    public int getMinimumDifference(TreeNode root) {
        list = new ArrayList<>();
        dfs(root);
        return result;
    }

    public void dfs(TreeNode node) {
        list.add(node.val);
        if (node.left != null) {
            for(int i = 0; i < list.size(); i++){
               result= Math.min(  Math.abs(list.get(i) - node.left.val) , result);
            }
            
            dfs(node.left);
        }
        if (node.right != null) {
            for(int i = 0; i < list.size(); i++){
                result= Math.min(  Math.abs(list.get(i) - node.right.val) , result);
            }
            dfs(node.right);
        }
    }
}