unique paths

int uniquePaths(int m, int n)
{
    vector<vector<int>> record((m + 1), vector<int>(n + 1));
    function<int(int, int)> getPathsNum;
    getPathsNum = [&](int a, int b)
    {
        if (a == 0 || b == 0){
            return 0;
        }
        if (a == 1 || b == 1){
            return 1;
        }
        int paths_num = record[a][b];
        if (paths_num != 0){
            return paths_num;
        }
        int&& result = getPathsNum(a - 1, b) + getPathsNum(a, b - 1);
        record[a][b] = result;
        return result;
    };
    return getPathsNum(m, n);
}