Validate Binary Search

这道题很简单，就是中序遍历，得到list然后valid数字排序即可

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    public boolean isValidBST(TreeNode root) {
        // write your code here
        ArrayList<Integer> rst = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curNode = root;
        while (curNode != null || !stack.empty()) {
            while (curNode != null) {
                stack.push(curNode);
                curNode = curNode.left;
            }
            curNode = stack.peek();
            stack.pop();
            rst.add(curNode.val);
            curNode = curNode.right;
        }
        
        for (int i = 1; i < rst.size(); i++) {
            if (rst.get(i) <= rst.get(i -1)) {
                return false;
            }
        }
        return true;
    }
}