98. Validate Binary Search Tree

这题要多想，首相二叉平衡树的判断一定要从全局来看，分别大于小于某个数！！！，第二点，我被最大最小值卡了，所以就用了long long int然后改最大最小值，2刷刷好看一点的代码，思路要更加清晰

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool mark;    void findit(TreeNode* root, long long int minn, long long int maxx){        if(root == NULL) return ;        if(mark == false) return ;        if(!(root -> val < maxx && root -> val > minn)){            mark = false;            return ;        }        if(root -> left != NULL){            if(root -> left -> val >= root -> val){                mark = false;                return ;            }            findit(root -> left, minn, root -> val);        }        if(root -> right != NULL){            if(root -> right -> val <= root -> val){                mark = false;                return ;            }            findit(root -> right, root -> val, maxx);        }        return ;    }    bool isValidBST(TreeNode* root) {        mark = true;        if(root == NULL) return true;        findit(root, -2147483649, 2147483648);        return mark;    }};