2. 两数相加

https://leetcode-cn.com/problems/add-two-numbers/

• 模拟

  public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
      ListNode head=null,tail=null;
      int carry=0;
      while (l1!=null||l2!=null){
          int n1 = l1!=null?l1.val:0;
          int n2 = l2!=null?l2.val:0;
          int sum = n1+n2+carry;
          if(head==null){
              head=tail=new ListNode(sum%10);
          }else{
              tail.next = new ListNode(sum%10);
              tail = tail.next;
          }
          carry = sum/10;
          if(l1!=null){
              l1=l1.next;
          }
          if(l2!=null){
              l2=l2.next;
          }
      }
      if(carry>0){
          tail.next = new ListNode(carry);
      }
      tail.next=null;
      return head;
  }
  

• 递归

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    return dfs(l1,l2,0);
}

private ListNode dfs(ListNode l1, ListNode l2, int i) {
    if(l1==null&&l2==null&&i==0) return null;
    int sum = (l1!=null?l1.val:0)+(l2!=null?l2.val:0)+i;
    ListNode node = new ListNode(sum%10);
    node.next = dfs(l1==null?null:l1.next,l2==null?null:l2.next,sum/10);
    return node;
}