LeetCode 2. Add Two Numbers ( C ++ )

題目為給定兩個鏈結串列，進行加法，回傳他們計算後的值(同樣以鏈結串列回傳)，數字由左而右依序進位。

原文題目如下

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

You may assume the two numbers do not contain any leading zero, except the number 0 itself.



Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]
Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

我採用的策略為，在較長的鏈結串列進行計算，結束後直接回傳該串列，以減少記憶體的使用量。

第一步先計算兩個鏈結串列的長度，並把 mainptr 指到較長的那個， optr 指到較短的那個，以 carry 變數計算進位值。

下方為我的程式碼

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *mainptr, *optr, *run;
        int carry = 0;
        if( CountList(l1) > CountList(l2) ){
            mainptr = l1;
            optr = l2;
        }
        else{
            mainptr = l2;
            optr = l1;
        }
        run = mainptr;

        while(optr != NULL || carry!=0){
            
            if(optr != NULL){
                carry += optr->val;
                optr = optr->next;
            }
            
            if(run != NULL){
                carry += run->val;
                run->val = carry % 10;
                if(run->next != NULL){
                    run = run->next;
                }
                else{
                    if(carry / 10){
                        run->next = new ListNode(carry/10);
                        run = run->next;
                        return mainptr;
                    }
                }
            }
            carry = carry / 10;
        }
        return mainptr;
    }
private:
    int CountList(ListNode *input){
        ListNode *temp = input;
        int counter = 0;
        while(temp!=NULL){
            counter ++;
            temp = temp->next;
        }
        return counter;
    }
};

下方為時間與空間之消耗

Runtime: 28 ms, faster than 62.33% of C++ online submissions for Add Two Numbers.

Memory Usage: 70.9 MB, less than 91.20% of C++ online submissions for Add Two Numbers.

下一題連結 : LeetCode 3. Longest Substring Without Repeating Characters ( C )-Medium

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