java 98.验证二进制搜索树（＃1递归）.java

Posted 2021-05-08

/**
 * 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) {
        TreeNode cur = root, pre = null;
        while (cur != null) {
            if(cur.left == null) {
                if (pre != null && cur.val <= pre.val) return false;
                pre = cur;
                cur = cur.right;
            } else {
                TreeNode temp = cur.left;
                while (temp.right != null && temp.right != cur) {
                    temp = temp.right;
                }
                if (temp.right == null) {
                    temp.right = cur;
                    cur = cur.left;
                } else {
                    if (pre != null && cur.val <= pre.val) return false;
                    temp.right = null;
                    pre = cur;
                    cur = cur.right;
                }
            }
        }
        return true;
    }
}

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root == null || (root.left == null && root.right == null)){
            return true;
        }
        Stack<TreeNode> s = new Stack<TreeNode>();
        TreeNode cur = root;
        TreeNode pre = null;
        while(true) {
            while (cur != null) {
                s.add(cur);
                cur = cur.left;
            }
            if (s.isEmpty()) {
                break;
            }
            cur = s.pop();
            if(pre != null && pre.val >= cur.val) {
                return false;
            }
            pre = cur;
            cur = cur.right;
            
        }
        return true;  
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    public boolean isValidBST(TreeNode root, long minVal, long maxVal) {
        if (root == null) return true;
        if (root.val >= maxVal || root.val <= minVal) return false;
        return isValidBST(root.left, minVal, root.val) && isValidBST(root.right, root.val, maxVal);
    }
}