中序遍历，从小到大

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return bool布尔型
     */
    public boolean isValidBST (TreeNode root) {
        // write code here
        // 中序遍历， 从小到大
        if (root == null) return true;
        LinkedList<TreeNode> st = new LinkedList<>();
        TreeNode temp = root;
        int pre = -Integer.MIN_VALUE;

        while (!st.isEmpty() || temp != null) {
            while (temp != null) {
                st.push(temp);
                temp = temp.left;
            }
            TreeNode t = st.poll();
            int now = t.val;
            if (pre > now) return false;
            pre = now;
            temp = t.right;
        }
        return true;
    }
}