問題描述

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

大意就是兩個連結串列相加，輸出結果連結串列。

思路

模擬平時列豎式做加法運算的過程即可。有幾個點需要考慮：

• 進位的儲存。因為最大隻能是 9 + 9 = 18，所以進位只可能為 0 或 1
• 2位整數取最低位。直接除以10即可。
• 最高位的進位。 如 L1 = 9, L2 = 1, 這時結果表L3 = 10, 比L1, L2多一個結點

Go實現

用時26ms, 擊敗了85%(用golang的)人

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    // 進位值, 只可能為0或1
    promotion := 0

    // 結果表的頭結點
    var
 
 head *ListNode
    // 儲存結果表的尾結點
    var rear *ListNode
    for nil != l1 || nil != l2 {
        sum := 0
        if nil != l1 {
            sum += l1.Val
            l1 = l1.Next
        }
        if nil != l2 {
            sum += l2.Val
            l2 = l2.Next
        }

        sum += promotion
        promotion = 0
 


        if sum >= 10 {
            promotion = 1
            sum = sum % 10
        }

        node := &ListNode{
            sum,
            nil,
        }

        if nil == head {
            head = node
            rear = node
        } else {
            rear.Next = node
            rear = node
        }

    }

    if promotion > 0 {
        rear.Next = &ListNode{
            promotion,
            nil,
        }
    }

    return head
}