参考答案

//递归式求最大最小值，时间复杂度O(n) 
void findminmax(treenode* tree, int& maxval, int& minval) {
    if (maxval < tree->val) {
        maxval = tree->val;
    }
    if (minval > tree->val) {
        minval = tree->val;
    }
    if (tree->left != NULL)
        findminmax(tree->left, maxval, minval);
    if (tree->right != NULL)
        findminmax(tree->right, maxval, minval);
}