[leedcode 98] Validate Binary Search Tree

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.

  /**
   * Definition for a binary tree node.
   * public class TreeNode {
   *     int val;
   *     TreeNode left;
   *     TreeNode right;
   *     TreeNode(int x) { val = x; }
   * }
   */
  public class Solution {
      public boolean isValidBST(TreeNode root) {
          //[2147483647]
          //注意，因为root的val是int型的，其可能为最大值或最小值，因此一开始的设置最大或最小要使用long型，注意函数参数的类型
          //本题非常巧妙使用最大值和最小值限制根节点的范围。最关键是对isvalid的正确定义，代表root的值是否在min和max中间
          
          //本题另一个解法是：将二叉树中序遍历，然后检测是否是递增数组
          return isvalid(root,(long)Integer.MIN_VALUE-1,(long)Integer.MAX_VALUE+1);
      }
      public boolean isvalid(TreeNode root,long min,long max){
          if(root==null){
              return true;
          }
          long val=(long)root.val;
          if(val>min&&val<max){
              return isvalid(root.left,min,val)&&isvalid(root.right,val,max);
          }else return false;
      }
  }