Leetcode 2 Add Two Numbers

Add two numbers

Question

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 contain a single digit. Add the two numbers and return it as a linked list.

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

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

 

 

Algorithms

The pseudocode is as following:

• Initialize current node to dummy head of the returning list.

• Initialize carry to 0.

• Initialize p and q to head of L_1 and L_2 respectively.

• Loop through lists ​ and ​ until you reach both ends.

  • Set x to node p's value. If p has reached the end of ​, set to 0.

  • Set y to node q's value. If q has reached the end of ​, set to 0.

  • Set sum = x + y + carry.

  • Update carry = sum / 10.

  • Create a new node with the digit value of ​ and set it to current node's next, then advance current node to next.

  • Advance both p and q.

• Check if carry = 1, if so append a new node with digit 1 to the returning list.

• Return dummy head's next node.

 

C++ code

坑1：链表每次初始化一个结点都要把next置为NULL；

坑2：输入[5],[5]的时候；

坑3：输入[1],[9,9]的时候。

 

/**
- Definition for singly-linked list.
- struct ListNode {
- int val;
- ListNode *next;
- ListNode(int x) : val(x), next(NULL) {}
- };
  */
  
  class Solution 
  {
  public:
  ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) 
  {
     ListNode *p = l1;
     ListNode *q = l2;
     int sum;
     int flag = 0;
​
    // new list to be returned
    ListNode *result = (ListNode*)malloc(sizeof(ListNode));
    ListNode *t = result;
​
    // process the first node alone
    sum = p->val + q->val;
    if(sum >= 10)
    {
        t->val = sum-10;
        flag = 1;
    }           
    else
        t->val = sum;
        
    p = p->next;
    q = q->next;
    while(p!=NULL && q!=NULL)
    {
        ListNode *next = (ListNode*)malloc(sizeof(ListNode));
        next->next = NULL;
        t->next = next;
        t = next;
​
        sum = p->val + q->val;
        if(flag == 1)
        {
            sum = sum + 1;
            flag = 0;
        }
​
        if(sum >= 10)
        {
            t->val = sum-10;
            flag = 1;
        }           
        else
            t->val = sum;
            
        p = p->next;
        q = q->next;
        sum = 0;
    }
​
    if(p==NULL && q!=NULL)
    {
        while(q!=NULL)
        {
            ListNode *next = (ListNode*)malloc(sizeof(ListNode));
            next->next = NULL;
            t->next = next;
            t = next;
​
            sum = q->val;
            if(flag == 1)
            {
                sum = sum + 1;
                flag = 0;
            }
​
            if(sum >= 10)
            {
                t->val = sum-10;
                flag = 1;
            }           
            else
                t->val = sum;
​
            q = q->next;
        }
    }
​
    if(p!=NULL && q==NULL)
    {
        while(p!=NULL)
        {
            ListNode *next = (ListNode*)malloc(sizeof(ListNode));
            next->next = NULL;
            t->next = next;
            t = next;
​
            sum = p->val;
            if(flag == 1)
            {
                sum = sum + 1;
                flag = 0;
            }
​
            if(sum >= 10)
            {
                t->val = sum-10;
                flag = 1;
            }           
            else
                t->val = sum;
​
            p = p->next;
        }
    }
​
    if(p==NULL && q==NULL && flag==1)
    {
        ListNode *next = (ListNode*)malloc(sizeof(ListNode));
        next->next = NULL;
        t->next = next;
        t = next;
        t->val = 1;
    }
​
    return result;
​
    }
};