leecode 98. 验证二叉搜索树

 

 

    public static boolean isValidBST(TreeNode root) {
        if(root==null)
            return true;
        return dfs(root, -Double.MAX_VALUE, Double.MAX_VALUE);  // 注意Double.MIN_VALUE是接近0的正数
    }
    public static boolean dfs(TreeNode root, double min_bound, double max_bound){   //设置两个参数，一个最大值，一个最小值
        if (root == null) return true;
        if (root.val >= max_bound || root.val <= min_bound) return false;
        return dfs(root.left, min_bound, root.val) && dfs(root.right, root.val, max_bound);
    }

 官方

class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean isValidBST(TreeNode node, long lower, long upper) {
        if (node == null) {
            return true;
        }
        if (node.val <= lower || node.val >= upper) {
            return false;
        }
        return isValidBST(node.left, lower, node.val) && isValidBST(node.right, node.val, upper);
    }
}