java 445.添加两个数字II（递归）.java

Posted 2021-05-08

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Deque<Integer> stack1 = new ArrayDeque<Integer>();
        Deque<Integer> stack2 = new ArrayDeque<Integer>();
        
        while(l1 != null) {
            stack1.push(l1.val);
            l1 = l1.next;
        }
        while(l2 != null) {
            stack2.push(l2.val);
            l2 = l2.next;
        }
        
        int sum = 0;
        ListNode newHead = new ListNode(-1);
        while(!stack1.isEmpty() || !stack2.isEmpty() ) {
            if(!stack1.isEmpty()) sum += stack1.pop();
            if(!stack2.isEmpty()) sum += stack2.pop();
            newHead.val = sum % 10;
            ListNode cur = new ListNode(sum / 10);
            cur.next = newHead;
            newHead = cur;
            sum /= 10;
        }
        return newHead.val == 0 ? newHead.next : newHead;
    }
}

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int size1 = getLength(l1);
        int size2 = getLength(l2);
        ListNode head = new ListNode(1);
        // Make sure l1.length >= l2.length
        head.next = size1 < size2 ? helper(l2, l1, size2 - size1) : helper(l1, l2, size1 - size2);
        // Handle the first digit
        if (head.next.val > 9) {
            head.next.val = head.next.val % 10;
            return head;
        }
        return head.next;
    }
    // get length of the list
    public int getLength(ListNode l) {
        int count = 0;
        while(l != null) {
            l = l.next;
            count++;
        }
        return count;
    }
    // offset is the difference of length between l1 and l2
    public ListNode helper(ListNode l1, ListNode l2, int offset) {
        if (l1 == null) return null;
        // check whether l1 becomes the same length as l2
        ListNode result = offset == 0 ? new ListNode(l1.val + l2.val) : new ListNode(l1.val);
        ListNode post = offset == 0 ? helper(l1.next, l2.next, 0) : helper(l1.next, l2, offset - 1);
        // handle carry 
        if (post != null && post.val > 9) {
            result.val += 1;
            post.val = post.val % 10;
        }
        // combine nodes
        result.next = post;
        return result;
    }