[98. Validate Binary Search Tree] #leetcode

98_0827.java

class Solution {
    public boolean isValidBST(TreeNode root) {
        return dfs(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    private boolean dfs(TreeNode node, long left, long right) {
        if ( node == null ) return true;
        
        return left < node.val && right > node.val && dfs(node.left, left, node.val) && dfs(node.right, node.val, right);
    }
}