Leetcode - 21 - Merge Two Sorted Lists (easy)

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博客主要围绕合并两个有序链表展开，提出将其合并为一个新的有序链表的问题。给出了递归和迭代两种解决方案，递归方案通过选择头节点后递归合并剩余链表解决；迭代方案包括使用虚拟节点连接最小节点和将一个链表合并到另一个链表中，还给出了Java和Python代码示例。

Merge two sorted linked lists and return it as a new sorted list. The new list should be made by splicing together the nodes of the first two lists.

Solutions:

(1) Recursive:

The recursive function mergeTwoLists(ListNode l1, ListNode l2) mean merged l1 and l2. So, with the head being chosen, merge the left list and another list recursively will solve the problem.

Java:

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;
		}
}

// https://leetcode.com/problems/merge-two-sorted-lists/discuss/9715/Java-1-ms-4-lines-codes-using-recursion

Python:

def mergeTwoLists2(self, l1, l2):
    if not l1 or not l2:
        return l1 or l2
    if l1.val < l2.val:
        l1.next = self.mergeTwoLists(l1.next, l2)
        return l1
    else:
        l2.next = self.mergeTwoLists(l1, l2.next)
        return l2

# https://leetcode.com/problems/merge-two-sorted-lists/discuss/9735/Python-solutions-(iteratively-recursively-iteratively-in-place).

C++:

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(l2->next, l1);
            return l2;
        }
    }
};

// This solution is not a tail-recursive, the stack will overflow while the list is too long :) http://en.wikipedia.org/wiki/Tail_call
// https://leetcode.com/problems/merge-two-sorted-lists/discuss/9713/A-recursive-solution

(2) Iterative:

(2.1) use a dummy node and link the smallest-val node after the previous linked node

Java:

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        else if(l2 == null) return l1;
        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;
        while(l1 != null && l2!= null){
            if(l1.val <= l2.val){
                curr.next = l1;
                l1 = l1.next;
            }else {
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }
        curr.next = l1 == null? l2:l1;
        return dummy.next;
    }

// by davidluoyes https://leetcode.com/problems/merge-two-sorted-lists/discuss/9715/Java-1-ms-4-lines-codes-using-recursion

Python:

def mergeTwoLists1(self, l1, l2):
    dummy = cur = ListNode(0)
    while l1 and l2:
        if l1.val < l2.val:
            cur.next = l1
            l1 = l1.next
        else:
            cur.next = l2
            l2 = l2.next
        cur = cur.next
    cur.next = l1 or l2
    return dummy.next

# https://leetcode.com/problems/merge-two-sorted-lists/discuss/9735/Python-solutions-(iteratively-recursively-iteratively-in-place).

(2.2) Merge l2 into l1

use p1 to save the previous node in l1

Java:

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null)  return l2;
        if(l2 == null)  return l1;
        ListNode p1 = null;
        ListNode head = l1;
        //Absorb l2 inside of l1
        while(l1 != null && l2 != null) {
            if(l1.val <= l2.val) {
                p1 = l1;
                l1 = l1.next;
            } else {
                //l2 < l1. So insert p1 -> l2 -> l1; make new p1 as l2, retain l1. and make l2 as old l2.next
                if(p1 != null) {
                    p1.next = l2;
                } else {
                    head = l2;
                }
                ListNode saveL2Next = l2.next;
                l2.next = l1;
                p1 = l2;
                l2 = saveL2Next;
            }
        }
        if(l1 == null)  p1.next = l2;
        //if l2 is null, do nothing, we have absorbed it inside l1
        return head;
    }

// by ftp1234 https://leetcode.com/problems/merge-two-sorted-lists/discuss/9715/Java-1-ms-4-lines-codes-using-recursion

Python:

def mergeTwoLists(self, l1, l2):
    if None in (l1, l2):
        return l1 or l2
    dummy = cur = ListNode(0)
    dummy.next = l1
    while l1 and l2:
        if l1.val < l2.val:
            l1 = l1.next
        else:
            nxt = cur.next
            cur.next = l2
            tmp = l2.next
            l2.next = nxt
            l2 = tmp
        cur = cur.next
    cur.next = l1 or l2
    return dummy.next

# https://leetcode.com/problems/merge-two-sorted-lists/discuss/9735/Python-solutions-(iteratively-recursively-iteratively-in-place).