Main98.java

package HOT100;

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.LinkedList;

public class Main98 {
    public boolean isValidBST(TreeNode root) {
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        double inorder = -Double.MAX_VALUE;

        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            // 如果中序遍历得到的节点的值小于等于前一个 inorder，说明不是二叉搜索树
            if (root.val <= inorder) {
                return false;
            }
            inorder = root.val;
            root = root.right;
        }
        return true;
    }
}

class Main98_1 {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean isValidBST(TreeNode node, long lower, long upper) {
        if (node == null) {
            return true;
        }
        if (node.val <= lower || node.val >= upper) {
            return false;
        }
        return isValidBST(node.left, lower, node.val) && isValidBST(node.right, node.val, upper);
    }
}

class Main98_2 {
    public boolean isValidBST(TreeNode root) {
        Deque<TreeNode> stack = new ArrayDeque<>();
        double inorder = -Double.MAX_VALUE;
        TreeNode p = root, tmp = null;
        while (!stack.isEmpty() || p != null) {
            while (p != null) {
                stack.push(p);
                p = p.left;
            }
            if(!stack.isEmpty()) {
                p = stack.pop();
                // 如果中序遍历得到的节点的值小于等于前一个 inorder，说明不是二叉搜索树
                if (p.val <= inorder) {
                    return false;
                }
                inorder = p.val;
                p = p.right;
            }
        }
        return true;
    }

    public boolean isValidBST2(TreeNode root) {
        return isValidBST(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    private boolean isValidBST(TreeNode root, int lower, int upper) {
        if(root == null) {
            return true;
        }
        if(root.val < lower || root.val > upper) {
            return false;
        }
        return isValidBST(root.left, lower, root.val) && isValidBST(root.right, root.val, upper);
    }
}