LeetCode - Medium - Add Two Numbers

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.

thinking

加法运算规则，指针的操作

solution

public class AddTwoNumbers {

	public class ListNode {
		int val;
		ListNode next;

		ListNode(int x) {
			val = x;
		}
		
		[@Override](https://my.oschina.net/u/1162528)
		public String toString() {
			StringBuilder sb = new StringBuilder();
			sb.append(val);
			ListNode pointer = next;
			while(pointer != null) {
				sb.append("->");
				sb.append(pointer.val);
				pointer = pointer.next;
			}
			
			return sb.toString();
		}
		
		/* 
		 * 借助toString 判断链表相等
		 * [@see](https://my.oschina.net/weimingwei) java.lang.Object#toString()
		 */
		[@Override](https://my.oschina.net/u/1162528)
		public boolean equals(Object obj) {
			if(!(obj instanceof ListNode))
				return false;
			
			return this.toString().equals(((ListNode)obj).toString());
		}

	}

	//假设l1,l2都是合法参数，返回新的链表
	public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
		
		//进位
		int carry = 0;
		
		ListNode head = null, tail = null;
		
		while(l1 != null || l2 != null) {
			
			int a = l1 != null ? l1.val : 0 ;
			int b = l2 != null ? l2.val : 0;
			
			int sum = a + b + carry;
			carry = sum / 10;
			
			ListNode tmp = new ListNode(sum % 10);
			
			if(head == null && tail == null) {
				head = tail = tmp;
			}else {
				tail.next = tmp;
				tail = tail.next;
			}
			
			if(l1 != null) {
				l1 = l1.next;
			}
			if(l2 != null) {
				l2 = l2.next;
			}
		}
		
		if(carry != 0) {
			tail.next = new ListNode(carry);
			tail = tail.next;
		}
		
		return head;
	}
	
	//辅助,非负数逆序队列,用队列实现
	public ListNode transform(int from) {
		
		//个位
		int digit = from % 10;
		ListNode tail = new ListNode(digit);
		ListNode head = tail;
		from = (from - digit) / 10; 
		
		while(from > 0) {
			digit = from % 10;
			tail.next = new ListNode(digit);
			tail = tail.next;
			from = (from - digit) / 10; 
		}
		
		return head;
	}
	
	@//Test
	public void testTransform() {		
		assertEquals("1", transform(1).toString());
		assertEquals("9->8->7->6->9->8->7->6", transform(67896789).toString());
		assertEquals("3->2->1", transform(123).toString());
		assertEquals(transform(123).toString(), transform(123).toString());
	}
	
	@//Test
	public void testAddTwoNumbers() {
		assertEquals("2->2->2", addTwoNumbers(transform(111), transform(111)).toString());
		assertEquals("7->0->8", addTwoNumbers(transform(342), transform(465)).toString());
		assertEquals("0->0->0->1", addTwoNumbers(transform(990), transform(10)).toString());
	}
}

reference

Add Two Numbers