size (); ++ i ) { if ( i!= 0 && nums [ i ] == nums [ i - 1 ]) continue ; unordered_set < int > seen ; for ( int j = i + 1 ; j < nums.
!= j + 1 && nums [ k ] == nums [ k - 1 ]) continue ; if ( nums [ i ] + nums [ j ] + nums [ k ] == 0 ) { output.