백준 14494 자바 - 다이나믹이 뭐예요?

// 다이나믹이 뭐예요?
package Silver_III_3;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Ex14494 {
    static long[][] dp;

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

        int n = Integer.parseInt(st.nextToken());
        int m = Integer.parseInt(st.nextToken());

        dp = new long[n+1][m+1];
        for (int i = 0; i <= n; i++) {
            for (int j = 0; j <= m; j++) {
                dp[i][j] = -1;
            }
        }

        System.out.println(dp(n, m) % 1000000007);
    }

    static long dp(int n, int m) {
        if (n == 0 || m == 0) { return 0; }
        if (n == 1 || m == 1) { return 1; }
        if (dp[n][m] != -1) { return dp[n][m]; }
        long res = (dp(n, m-1) + dp(n-1, m) + dp(n-1, m-1)) % 1000000007;
        dp[n][m] = res;
        return res;
    }
}