If the characters are equal then look for out of bound index
if ( i - 1 >= 0 ) { dp [ i ] [ j ] += dp [ i - 1 ] [ j ] ; } if ( j + 1 <= n - 1 ) { dp [ i ] [ j ] += dp [ i ] [ j + 1 ] ; } if ( i - 1 < 0 j + 1 >= n ) {