題目445. Add Two Numbers II

You are given two linked lists representing two non-negative numbers. 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

思路:類似用頭插法建立鏈表

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null || l2 == null){
            return l1 == null ? l1 : l2;
        }
        Stack<ListNode> stack1 = new Stack<ListNode>();
        Stack<ListNode> stack2 = new Stack<ListNode>();
        saveList(stack1,l1);
        saveList(stack2,l2);
        
        ListNode tempHead = new ListNode(1);
        ListNode node1 = null;
        ListNode node2 = null;
        int rise = 0;
        while(!stack1.empty() && !stack2.empty()){
            node1 = stack1.pop();
            node2 = stack2.pop();
            int sum = node1.val + node2.val + rise;
            rise = sum / 10;
            sum %= 10;
            ListNode newNode = new ListNode(sum);
            newNode.next = tempHead.next;
            tempHead.next = newNode;
        }
        
        while(!stack1.empty()){
            node1 = stack1.pop();
            int sum = node1.val + rise;
            rise = sum / 10;
            sum %= 10;
            ListNode newNode = new ListNode(sum);
            newNode.next = tempHead.next;
            tempHead.next = newNode;
        }
        
         while(!stack2.empty()){
            node2 = stack2.pop();
            int sum = node2.val + rise;
            rise = sum / 10;
            sum %= 10;
            ListNode newNode = new ListNode(sum);
            newNode.next = tempHead.next;
            tempHead.next = newNode;
        }
        
        if(rise != 0){
            ListNode newNode = new ListNode(rise);
            newNode.next = tempHead.next;
            tempHead.next = newNode;
        }
        return tempHead.next;
    }

    private void saveList(Stack<ListNode> stack, ListNode head){
        ListNode node = head;
        while(node != null){
            stack.push(node);
            node = node.next;
        }
    }
}