class Solution {
    TreeNode pre;
    int result = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
       if(root==null)return 0;
       traversal(root);
       return result;
    }
    public void traversal(TreeNode root){
        if(root==null)return;
        //左
        traversal(root.left);
        //中
        if(pre!=null){
            result = Math.min(result,root.val-pre.val);
        }
        pre = root;
        //右
        traversal(root.right);
    }
}

class Solution {
    public int[] findMode(TreeNode root) {
        TreeNode pre = null;
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> result = new ArrayList<>();
        int maxCount = 0;
        int count = 0;
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                stack.push(cur);
                cur =cur.left;
            }else {
                cur = stack.pop();

                if (pre == null || cur.val != pre.val) {
                    count = 1;
                }else {
                    count++;
                }

                if (count > maxCount) {
                    maxCount = count;
                    result.clear();
                    result.add(cur.val);
                }else if (count == maxCount) {
                    result.add(cur.val);
                }
                pre = cur;
                cur = cur.right;
            }
        }
        return result.stream().mapToInt(Integer::intValue).toArray();
    }
}

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || root == p || root == q) { 
            return root;
        }

        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);

        if(left == null && right == null) { 
            return null;
        }else if(left == null && right != null) { 
            return right;
        }else if(left != null && right == null) { 
            return left;
        }else { 
            return root;
        }
    }
}