LeetCode 21. Merge Two Sorted Lists


Title Description (simple difficulty)

LeetCode 21. Merge Two Sorted Lists

Merge two ordered lists.

Solution one iteration

Traverse two linked lists.

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

Time complexity: O (M + n).

Spatial complexity: O (1).

Solution 2 recursion

Reference here

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;

Time complexity:

Spatial complexity:


Recursion looks, two words, elegant! But as for the time complexity and space complexity of recursion, let’s leave a hole first.

See leetcode.wang for more details.

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