Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node's key.
  • The right subtree of a node contains only nodes with keys greater than the node's key.
  • Both the left and right subtrees must also be binary search trees.

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这道题的测试用例有点恶心，最后直接全部用long才没有溢出了

 1 public class Solution {
 2     public boolean isValidBST(TreeNode root) {
 3        return isBST(root, ((long)Integer.MIN_VALUE) - 1, ((long)Integer.MAX_VALUE) + 1); 
 4     }
 5     private boolean isBST(TreeNode root, long leftVal,long rightVal){
 6         if(root == null)
 7             return true;
 8         
 9         return ((long)root.val) > leftVal && ((long)root.val) < rightVal && isBST(root.left, leftVal, ((long)root.val))
10                 && isBST(root.right, ((long)root.val), rightVal);
11     }
12 }