LeetCode-445. Add Two Numbers II

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问题描述

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

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

Example :

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Input: l1 = [7,2,4,3], l2 = [5,6,4]
Output: [7,8,0,7]

Constraints:

  • The number of nodes in each linked list is in the range [1, 100].
  • 0 <= Node.val <= 9
  • It is guaranteed that the list represents a number that does not have leading zeros.

解答

方法一：反转链表

先将两个链表反转，然后从前往后逐个节点相加，最后再将结果链表反转

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/*
 * @Description: 445. Add Two Numbers II
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
const addTwoNumbers = function(l1, l2) {
  let reL1 = reverseList(l1)
  let reL2 = reverseList(l2)
  let dummy = new ListNode()
  let cur = dummy
  let carry = 0

  while (reL1 !== null || reL2 !== null || 1 === carry) {
    let sum = 0
    if (reL1 !== null) {
      sum += reL1.val
      reL1 = reL1.next
    }
    if (reL2 !== null) {
      sum += reL2.val
      reL2 = reL2.next
    }
    sum += carry
    carry = Math.floor(sum / 10)
    cur.next = new ListNode(sum % 10)
    cur = cur.next
  }
  return reverseList(dummy.next)
}

function reverseList (head) {
  if (head === null) {
    return head
  }
  let pre = null
  let cur = head
  while (cur !== null) {
    let temp = cur.next
    cur.next = pre
    pre = cur
    cur = temp
  }
  return pre
}
方法二：栈

利用栈先入后出的特性，将两个链表依次压入栈，然后从栈中取出每个节点相加

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/*
 * @Description: 445. Add Two Numbers II
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @description: 通过栈来解决
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
const addTwoNumbers = function(l1, l2) {
  let stack1 = []
  let stack2 = []
  let cur1 = l1
  let cur2 = l2
  while (cur1 !== null) {
    stack1.push(cur1)
    cur1 = cur1.next
  }
  while (cur2 !== null) {
    stack2.push(cur2)
    cur2 = cur2.next
  }

  let cur = null
  let carry = 0
  while (stack1.length || stack2.length || carry === 1) {
    let sum = 0
    if (stack1.length) {
      sum += stack1.pop().val
    }
    if (stack2.length) {
      sum += stack2.pop().val
    }
    sum += carry
    let newHead = new ListNode(sum % 10, cur)
    cur = newHead
    carry = Math.floor(sum / 10)
  }
  return cur
}