Amazon Tag

Add Two Numbers

• 问题: 给出两个链表分别表示倒序的非负整数,求两个整数的和

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null && l2 == null) {
            return null;
        }
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }
        int carry = 0;
        ListNode curr = new ListNode(0);
        ListNode dummy = curr;
        while (l1 != null && l2 != null) {
            curr.next = new ListNode((l1.val + l2.val + carry) % 10);
            carry = (l1.val + l2.val + carry) / 10;
            curr = curr.next;
            l1 = l1.next;
            l2 = l2.next;
        }
        while (l1 == null && l2 != null) {
            curr.next = new ListNode((l2.val + carry) % 10);
            carry = (l2.val + carry) / 10;
            curr = curr.next;
            l2 = l2.next;
        }
        while (l2 == null && l1 != null) {
            curr.next = new ListNode((l1.val + carry) % 10);
            carry = (l1.val + carry) / 10;
            curr = curr.next;
            l1 = l1.next;
        }
        if (carry != 0) {
            curr.next = new ListNode(carry);
        }
        
        return dummy.next;
    }
}

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

445. Add Two Numbers II

• 问题: 给出两个链表分别表示正序的非负整数,求两个整数的和
• 思路: 先reverse然后再直接求加 -> 长度不等的时候, 需要考虑进位匹配问题 —> 计算长度用0补位 或者是先将长的跳过 从同一起点开始reverse, 高位是在前面,计算完了还要reverse回去