The number of ways of reaching the last step from the previously taken step would be from either **(n-1)th** step or **(n-2)th** step. So the total number of ways of reaching the top would be the sum of number of ways of reaching **(n-1)** and **(n-2)th** step.

So the recursive version of this would be:

```
public int ClimbStairs(int n) {
if(n<0)
return 0; \\base case
else if(n == 0)
return 1;
return ClimbStairs(n-1) + ClimbStairs(n-2);
}
```

Converting it into a DP solution:

```
public int ClimbStairs(int n) {
int[] dp = new int[n+1];
dp[0] = 1;
dp[1] = 1;
for(int i=2;i<=n;i++){
dp[i] = dp[i-1] + dp[i-2];
}
return dp[n];
}
```