LeetCode98.java

package problems;

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

/**
 * 递归
 */
public class LeetCode98 {
    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);
    }
}

/**
 * 中序遍历非递归
 */
class LeetCode98_1 {
    public boolean isValidBST(TreeNode root) {
        Deque<TreeNode> stack = new LinkedList<>();
        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;
    }
}