1^k+2^k+...+n^k = sigma(P[k][l] * C[n+1][l+1]); l = 1,2,...k; 
其中C[i][j] 为组合数，C[i][j] = C[i-1][j] + C[i-1][j-1];
P[i][0] = 0;
P[i][1] = 1;
P[i][i] = 1 * 2 * ... * i = i !;
P[i][j] = j * (P[i-1][j-1] + P[i-1][j]);