Spring Authorization Server常见问题解答(FAQ)

目录

530.二叉搜索树的最小绝对差

 230.二叉搜索树中第K小的元素

 98.验证二叉搜索树

530.二叉搜索树的最小绝对差

class Solution {
    int ans,pre;//Pre保存前驱节点的值
    public int getMinimumDifference(TreeNode root) {
        ans = Integer.MAX_VALUE;
        pre = -1;//提示中说明节点值的范围是[0,105],所以初始化为-1
        midOrder(root);
        return ans;      
    }

    void midOrder(TreeNode root){
        if(root == null){
            return;
        }
        midOrder(root.left);
        if(pre == -1){
            pre = root.val;
        }else{
            ans = Math.min(ans, root.val - pre);
            pre = root.val;
        }
        midOrder(root.right);
    }
}

 230.二叉搜索树中第K小的元素

class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        while(root != null || !stack.isEmpty()){
            while(root != null){
                stack.push(root);//先把根存进去，然后先找左子树
                root = root.left;//左 根 右，持续进栈根节点，直到找到最左的节点
            }
            //循环结束条件是root为空，证明上一个root没有左节点，出栈
            root = stack.pop();
            --k;//遍历到一个数减一次
            if(k == 0){
                break;
            }
            root = root.right;//往节点右子树找了
        }
        return root.val;
    }
}

 98.验证二叉搜索树

class Solution {
    public boolean isValidBST(TreeNode root) {
        double pre = -Double.MAX_VALUE;;
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        while(root != null || !stack.isEmpty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if(root.val <= pre){//=也不符合二叉搜索树的要求
                return false;//不满足
            }
            pre = root.val;
            root = root.right;
        }
        return true;
    }
}