21. Merge Two Sorted Lists

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LeetCode

Approach 1: Recursion

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null) return l2;
        if(l2==null) return l1;
        
        if(l1.val<=l2.val){
            l1.next=mergeTwoLists(l1.next,l2);
            return l1;
        }else{
            l2.next=mergeTwoLists(l1,l2.next);
            return l2;
        }
    }
}

Approach 2: Iteration

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummyHead=new ListNode(-1);
        ListNode tmp=dummyHead;
        while(l1!=null && l2!=null){
            if(l1.val<=l2.val){
                tmp.next=l1;                
                l1=l1.next;
            }
            else{
                tmp.next=l2;
                l2=l2.next;
            }
            tmp=tmp.next;
        }  
        tmp.next=l1==null?l2:l1;
            
        return dummyHead.next;
    }
}

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