1
class Solution {
2
  public boolean isValidBST(TreeNode root) {
3
    return dfs(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
4
  }
5

6
  public boolean dfs(TreeNode root, int min, int max) {
7
    if (root.val < min
8
        || root.val > max
9
        || (root.val == Integer.MIN_VALUE && root.left != null)
10
        || (root.val == Integer.MAX_VALUE && root.right != null)) return false;
11
    boolean leftRight = true;
12
    if (root.left == null && root.right == null) return true;
13
    if (root.left != null) {
14
      leftRight = dfs(root.left, min, root.val - 1);
15
    }
16
    if (root.right != null) {
17
      leftRight = dfs(root.right, root.val + 1, max) && leftRight;
18
    }
19
    return leftRight;
20
  }
21
}