Validate Binary Search Tree

写道

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.

   解题思路 ： 利用BST中序遍历是排好序的！

   第一次做的时候思路错了，以为只要遍历的时候，加一个条件判断是否左右节点满足条件！

   那样的话，就可能存在根节点的右边小于它的节点或左边大于他的节点；

 

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        List<Integer> nodes = new ArrayList<Integer> ();
        return validateBST(root,nodes);
    }
    
    boolean validateBST(TreeNode root,List<Integer> nodes ) {
        if (root == null) return true;
        if (!validateBST(root.left,nodes))
            return false;
        int val = root.val;
        if (nodes.isEmpty()) nodes.add(val);
        else if (nodes.get(nodes.size() - 1) >= val) {
            return false;
        } {
            nodes.add(val);
        }
        if (!validateBST(root.right,nodes))
            return false;
        return true;
    }
}