445. Add Two Numbers II

/**
 * 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) {
      // put two numbers into stack separtely
      // when pop them out, we add two numbers and keep a carry for the next
      // digit, and push every sum into the stack 
      
      // after we done pushing the sum into the stack, 
      // we can pop them out and link them into a linked list 
      
      Deque<ListNode> stack1 = new ArrayDeque<>();
      Deque<ListNode> stack2 = new ArrayDeque<>();
      
      while(l1 != null) {
        stack1.push(l1);
        l1 = l1.next;
      }
      while( l2 != null) {
        stack2.push(l2);
        l2 = l2.next;
      }
      
      int carry = 0;
      Deque<Integer> stack = new ArrayDeque<>();
      while( !stack1.isEmpty() || !stack2.isEmpty()){
        int num1 = stack1.size() == 0 ? 0 : stack1.pop().val;
        int num2 = stack2.size() == 0 ? 0 : stack2.pop().val;
        
        int sum = carry + num1 + num2;
        carry = sum / 10;
        sum = sum % 10;
        stack.push(sum);
      }
      if( carry != 0){
        stack.push(carry);
      }
      
      
      ListNode dummy = new ListNode(0);
      ListNode cur = dummy;
      while(!stack.isEmpty()){
        int number = stack.pop();
        ListNode newListNode = new ListNode(number);
        cur.next = newListNode;
        cur = cur.next;
      }
      return dummy.next;
    }
}

 

////// others code 


public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        
        while(l1 != null) {
            s1.push(l1.val);
            l1 = l1.next;
        };
        while(l2 != null) {
            s2.push(l2.val);
            l2 = l2.next;
        }
        
        int sum = 0;
        ListNode list = new ListNode(0);
        while (!s1.empty() || !s2.empty()) {
            if (!s1.empty()) sum += s1.pop();
            if (!s2.empty()) sum += s2.pop();
            list.val = sum % 10;
            ListNode head = new ListNode(sum / 10);
            head.next = list;
            list = head;
            sum /= 10;
        }
        
        return list.val == 0 ? list.next : list;
    }
}

 

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

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