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func mergeTwoLists(_ list1: ListNode?, _ list2: ListNode?) -> ListNode? {
        var finalList: ListNode?
        var firstList: ListNode? = list1
        var secondList: ListNode? = list2
        var tmpListNode: ListNode?
        var cnt = 0
        while(firstList != nil && secondList != nil) {
            if firstList!.val <= secondList!.val {
                if cnt==0 {
                    tmpListNode = firstList
                    finalList = tmpListNode
                }else {
                    tmpListNode!.next = firstList
                    tmpListNode = firstList
                }
                firstList = firstList!.next
            }else {
                if cnt==0 {
                    tmpListNode = secondList
                    finalList = tmpListNode
                }else {
                    tmpListNode!.next = secondList
                    tmpListNode = secondList
                }
                secondList = secondList!.next
            }
            cnt += 1
        }
        while(firstList != nil) {
            if cnt==0 {
                tmpListNode = firstList
                finalList = tmpListNode
            }else {
                tmpListNode!.next = firstList
                tmpListNode = firstList
            }
            firstList = firstList!.next
            cnt += 1
        }
        while(secondList != nil) {
            if cnt==0 {
                tmpListNode = secondList
                finalList = tmpListNode
            }else {
                tmpListNode!.next = secondList
                tmpListNode = secondList
            }
            secondList = secondList!.next
            cnt += 1
        }
        return finalList
    }