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JavaScript

let mergeTwoLists = function(l1, l2) {

  let dummy = {
  	val: -1,
    next: null,
  }
  
  let head = dummy;
  
  while (l1 !== null && l2 !== null) {
    if (l1.val <= l2.val) {
      dummy.next = l1;
      l1 = l1.next;
    } else {
      dummy.next = l2;
      l2 = l2.next;
    }
    dummy = dummy.next;
  }

  if (l1 !== null) {
    dummy.next = l1;
  } else {
    dummy.next = l2;
  }

  return head.next;
};



const a = {
	val: 1,
  next: {
  	val: 3,
    next: null
  }
}

const b = {
	val: 2,
  next: {
  	val: 4,
    next: null
    }
}

console.log(mergeTwoLists(a,b));