[Programmers] 조합 nCr - Dynamic Programming

// 조합 nCr - Dynamic Programming 
private long combination(int n, int r){
if(n == r || r == 0){
return 1;
}
if(dp[n][r] > 0){
return dp[n][r];
}
dp[n][r] = combination(n-1,r-1) + combination(n-1, r);
return dp[n][r];
}