you give doubly link list contain node pointer previous pointer additional child pointer this child pointer point separate doubly link list contain special node these child list child produce multilevel datum structure show example belowGiven head level list flatten list node appear singlelevel doubly link list let curr node child list the node child list appear curr currnext flatten listReturn head flatten list the node list child pointer set null Example 1input head 123456nullnullnull78910nullnull1112output 123781112910456explanation the multilevel link list input shownAfter flatten multilevel link list becomesExample 2input head 12null3output 132explanation the multilevel link list input shownAfter flatten multilevel link list becomesExample 3input head Output Explanation there list input ConstraintsThe number Nodes exceed 10001 Nodeval 105 how multilevel link list represent test casesWe use multilevel link list Example 1 123456null 78910null 1112nullthe serialization level follows123456null78910null1112nullto serialize level add null level signify node connect upper node previous level the serialization becomes1 2 3 4 5 6 null null null 7 8 9 10 null null 11 12 nullmerge serialization level remove trailing null obtain123456nullnullnull78910nullnull1112