BOJ)10844 쉬운 계단 수(다시보기)

https://www.acmicpc.net/problem/10844

#include <iostream>
#include <algorithm>
using namespace std;
 
//다시보기
 
long long dp[101][10];
#define MOD 1000000000
 
int main()
{
    int n;
    cin >> n;
 
    
    for (int i = 1; i < 10; i++) { dp[1][i] = 1; }
 
    for (int i = 2; i <= n; i++) {
        for (int j = 0; j < 10; j++) {
            dp[i][j] = 0;
            if (j - 1 >= 0) dp[i][j] += dp[i - 1][j - 1];
            if (j + 1 <= 9) dp[i][j] += dp[i - 1][j + 1];
            dp[i][j] %= MOD;
            //dp[i][j] = dp[i - 1][j - 1] + dp[i-1][j+1];
        }
    }
 
    long long res = 0;
    for (int i = 0; i < 10; i++) res += dp[n][i];
    cout << res % MOD;
 
}
 
/*
dp[3]
0 - 1
dp[2]
1 - 0, 2
2 - 1, 3
3 - 2, 4
4 - 3, 5
5 - 4, 6
6 - 5, 7
7 - 6, 8
8 - 7, 9
9 - 8
*/