leetcode-98 验证二叉搜索树 Java

https://leetcode-cn.com/problems/validate-binary-search-tree/solution/yan-zheng-er-cha-sou-suo-shu-by-leetcode-solution/

方法一：中序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    long pre = Long.MIN_VALUE;
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }

        if (!isValidBST(root.left)) {
            return false;
        }

        if (root.val <= pre) {
            return false;
        }
        pre = root.val;
        return isValidBST(root.right);
    }
}

方法二：递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean helper(TreeNode node, Integer lower, Integer upper) {
        
        if (node == null) return true;
        int val = node.val;
        if (lower != null && val <= lower) return false;
        if (upper != null && val >= upper) return false;

        if (!helper(node.right, val, upper)) return false;
        if (!helper(node.left, lower, val)) return false;
        return true;
    }
    public boolean isValidBST(TreeNode root) {
        return helper(root, null, null);
    }
}

方法三：栈

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack();
        double inorder = -Double.MAX_VALUE;

        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.val <= inorder) return false;
            inorder = root.val;
            root = root.right;
        }
        return true;
    }
}