C++ 代码

/**
 * 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 isValidBST(TreeNode* root) {
        return dfs(root,INT_MIN,INT_MAX);   //INT_MAX = 2^31 - 1 =2147483647 
                                            //INT_MIN= - 2^31 = -2147483648
    }

    bool dfs(TreeNode* root, long long minv, long long maxv)
    {
        if (!root) return true;
        if (root->val<minv||root->val>maxv) return false;
        return dfs(root->left,minv,root->val-1ll)&&dfs(root->right,root->val+1ll,maxv);
        //LL代表long long, * 1LL是为了在计算时，把int类型的变量转化为long long，然后再赋值给longlong类型的变量
    }
};