text: DFS function to find ideal pairs
code: static void dfs ( int node ) { Ideal_pair += bit_q ( Math . max ( 1 , node - k ) , Math . min ( n , node + k ) ) ; bit_up ( node , 1 ) ; for ( int i = 0 ; i < al [ node ] . size ( ) ; i ++ ) dfs ( al [ node ] . get ( i ) ) ; bit_up ( node , - 1 ) ; }