Unique Binary Search Trees

int numTrees(int n) {
    vector<int> treeNumList(n + 1);
    function<int(int)> getTreeNum;
    getTreeNum = [&](int num)
    {
        if (num == 0 || num == 1){
            return 1;
        }
        int result = 0;
        if ((result = treeNumList[num]) != 0){
            return result;
        }
        bool const isOdd = (num % 2 != 0);
        const int k = isOdd? (num - 1) / 2 : num / 2;
        for (int i = 0; i != k; ++i){
            result += 2 * getTreeNum(i) * getTreeNum(num - i - 1);
        }
        if (isOdd){
            result += getTreeNum(k) * getTreeNum(k);
        }
        treeNumList[num] = result;
        return result;
    };
    return getTreeNum(n);
}