21. Merge Two Sorted Lists

Merge Two Sorted Lists - LeetCodeLeetCode

time: O(n + m)

space: O(n + m)

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
const dummy = new ListNode();
var mergeTwoLists = function(list1, list2) {
    dummy.next = null;

    let prev = dummy;
    // // do not need
    // let curr1 = list1; 
    // let curr2 = list2; 
    while (list1 !== null && list2 !== null) {
        if (list1.val <= list2.val) {
            prev.next = list1;
            prev = list1;
            list1 = list1.next;
        } else {
            prev.next = list2;
            prev = list2;
            list2 = list2.next;
        }
    }

    if (list1 !== null) {
        prev.next = list1;
    } 
    
    if (list2 !== null) {
        prev.next = list2;
    }

    return dummy.next;
};

do not need iteration

// as is

    while (curr1 !== null) {
        prev.next = curr1;
        prev = curr1;
        curr1 = curr1.next;
    }

    while (curr2 !== null) {
        prev.next = curr2;
        prev = curr2;
        curr2 = curr2.next;
    }
    
// to be    
    if (curr1 !== null) {
        prev.next = curr1;
    } 
    
    if (curr2 !== null) {
        prev.next = curr2;
    }