Leetcode 21 Solution

This article provides solution to leetcode question 21 (merge-two-sorted-lists)
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https://leetcode.com/problems/merge-two-sorted-lists

Thinking Process

This is a basic question. Do remember to leverage the dummy head trick to avoid any unnecessary head processing logic.

Time & Space Complexity

Assuming N is the average length of the string

  • Time complexity: O(N)
  • Space complexity: O(1)

Solution

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head = NULL;
        ListNode* p = NULL;

        while (l1 || l2)
        {
            int v = INT_MAX;
            ListNode* next = NULL;

            if (l1 && v >= l1->val)
            {
                next = l1;
                v = l1->val;
            }

            if (l2 && v >= l2->val)
            {
                next = l2;
                v = l2->val;
            }

            if (head == NULL)
                head = next;

            if (p != NULL)
                p->next = next;

            p = next;

            if (next == l1)
            {
                l1 = l1->next;
            }
            else if (next == l2)
            {
                l2 = l2->next;
            }
        }

        return head;
    }
};