换种写法
```cpp
int count_x = 0;

unordered_map<int,int> map_x;

int recur(int n) {
count_x++;

if (map_x.find(n) != map_x.end()) {
return map_x[n];
}

if (n < 3) {
map_x[n] = 1;
return map_x[n];
}

int next_1 = (map_x.find(n-1) == map_x.end()) ? recur(n-1) : map_x[n-1];
int next_2 = (map_x.find(n-2) == map_x.end()) ? recur(n-2) : map_x[n-2];
int next_3 = (map_x.find(n-3) == map_x.end()) ? recur(n-3) : map_x[n-3];

map_x[n] = next_1 * next_2 * next_3;
return map_x[n];
}
```