题目链接

层序遍历

/**
 * 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 getMinimumDifference(TreeNode root) {
        ArrayList<Integer> array = new ArrayList<>();
        Deque<TreeNode> queue = new LinkedList<TreeNode>();
        if(root == null){
            return 0;
        }
        queue.offer(root);
        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            array.add(node.val);
            if(node.left != null){
                queue.offer(node.left);
            }
            if(node.right != null){
                queue.offer(node.right);
            }
        }
        // 使用 Collections.sort(array) 对 ArrayList 进行排序
        Collections.sort(array);
        int min = Integer.MAX_VALUE;
        for(int i = 1; i < array.size(); i++){
           min = Math.min(min, array.get(i) - array.get(i - 1));
        }
        return min;
    }
}

在 Java 中，不能使用 array.sort() 的原因是 ArrayList 类没有直接提供 sort() 方法。应该使用Collections.sort() 或 List.sort() 方法对列表进行排序。

中序遍历

做麻烦了，二叉搜索树中序遍历可以直接得到有序序列！！！！

/**
 * 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 {
    ArrayList<Integer> array = new ArrayList();
    public int getMinimumDifference(TreeNode root) {
        traversal(root);
        if(array.size() < 2){
            return 0;
        }
        int min = Integer.MAX_VALUE;
        for(int i = 1; i < array.size(); i++){
            min = Math.min(min,array.get(i) - array.get(i - 1));
        }
        return min;
    }

    public void traversal(TreeNode root){
        if(root == null){
            return;
        }
        traversal(root.left);
        array.add(root.val);
        traversal(root.right);
    }
}