2. 两数相加

迭代

class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {ListNode h1 = l1, h2 = l2, dummy = new ListNode(-1, h1), d = dummy;int carry = 0, sum = 0;while(h1 != null && h2 != null){sum = h1.val + h2.val + carry;carry = sum / 10;h1.val = sum % 10;d.next = h1;d = d.next;h1 = h1.next;h2 = h2.next;}while(h1 != null){sum = h1.val + carry;carry = sum / 10;h1.val = sum % 10;d.next = h1;d = d.next;h1 = h1.next;}while(h2 != null){sum = h2.val + carry;carry = sum / 10;h2.val = sum % 10;d.next = h2;d = d.next;h2 = h2.next;}if(carry != 0){d.next = new ListNode(carry);}return dummy.next;}
}

递归

class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {return addTwoNumbers(l1, l2, 0);}ListNode addTwoNumbers(ListNode l1, ListNode l2, int carry) {if(l1 == null && l2 == null){if(carry != 0) return new ListNode(carry);else return null;}if(l1 == null) {int sum = l2.val + carry;l2.val = sum % 10;carry = sum / 10;l2.next = addTwoNumbers(null, l2.next, carry);return l2;}if(l2 == null){int sum = l1.val + carry;l1.val = sum % 10;carry = sum / 10;l1.next = addTwoNumbers(l1.next, l2, carry);return l1;}int sum = l1.val + l2.val + carry;l1.val = sum % 10;carry = sum / 10;l1.next = addTwoNumbers(l1.next, l2.next, carry);return l1;}
}