445：两数相加II

问题描述

这个是字节跳动面试题。
给你两个 非空 链表来代表两个非负整数。数字最高位位于链表开始位置。它们的每个节点只存储一位数字。将这两数相加会返回一个新的链表。

你可以假设除了数字 0 之外，这两个数字都不会以零开头。

进阶：

• 如果输入链表不能修改该如何处理？换句话说，你不能对列表中的节点进行翻转。

示例

输入：(7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
输出：7 -> 8 -> 0 -> 7

思路

读到数组里，然后从后往前相加就行了。效率最高（方法一）
用栈，因为是倒序的嘛，栈是个自然而然的思想。（方法二）

方法一

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}
//
//class TreeNode {
//    int val;
//    TreeNode left;
//    TreeNode right;
//    TreeNode(int x) { val = x; }
//}
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int countL1 = 0,countL2 = 0;
        ListNode tmp = l1;
        while(tmp != null){
            countL1++;
            tmp = tmp.next;
        }
        tmp = l2;
        while(tmp != null){
            countL2++;
            tmp = tmp.next;
        }
        tmp = l1; int i = 0;
        int[] L1 = new int[countL1];
        int[] L2 = new int[countL2];
        while(tmp != null){
            L1[i++] = tmp.val;
            tmp = tmp.next;
        }
        tmp = l2; i = 0;
        while(tmp != null){
            L2[i++] = tmp.val;
            tmp = tmp.next;
        }
        if(L1.length < L2.length){
            int[] t = L1;
            L1 = L2;
            L2 = t;
        }
        int remains = 0;
        /*数组相加*/
        int index1,index2;
        for(index1 = L1.length-1, index2 = L2.length-1; index1 > -1 && index2 > -1; index1--,index2--){
            int curSum = L1[index1]+L2[index2]+remains;
            remains = curSum/10;
            L1[index1] = curSum%10;
        }
        for(int j = index1; j > -1; j--){
            int curSum = L1[j] + remains;
            remains = curSum/10;
            L1[j] = curSum%10;
            if(remains == 0) break;
        }
        ListNode res = new ListNode(-1);
        ListNode cur = res;
        if(remains != 0){
            cur.next = new ListNode(1);
            cur = cur.next;
        }
        for(i = 0; i < L1.length; i++){
            cur.next = new ListNode(L1[i]);
            cur = cur.next;
        }
        return res.next;
    }
}

嗯。。有点丑，重构一下。

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int countL1 = getLength(l1),countL2 = getLength(l2);
        int[] L1 = getArr(l1,countL1), L2 = getArr(l2,countL2);
        if(L1.length < L2.length){
            int[] t = L1;
            L1 = L2;
            L2 = t;
        }
        int remains = getRemainsAndMergeArr(L1,L2);
        /*数组相加*/
        ListNode res = new ListNode(-1);
        ListNode cur = res;
        if(remains != 0){
            cur.next = new ListNode(1);
            cur = cur.next;
        }
        for(int i = 0; i < L1.length; i++){
            cur.next = new ListNode(L1[i]);
            cur = cur.next;
        }
        return res.next;
    }
    private int getLength(ListNode l){
        int res = 0;
        while(l != null){
            res++;
            l = l.next;
        }
        return res;
    }
    private int[] getArr(ListNode l, int length){
        int[] res = new int[length];
        int i = 0;
        while(l != null){
            res[i++] = l.val;
            l = l.next;
        }
        return res;
    }
    private int getRemainsAndMergeArr(int[] arr1, int[] arr2){
        int index1 = arr1.length-1, index2 = arr2.length-1;
        int remains = 0;
        for(index1 = arr1.length-1, index2 = arr2.length-1; index1 > -1 && index2 > -1; index1--,index2--){
            int curSum = arr1[index1]+arr2[index2]+remains;
            remains = curSum/10;
            arr1[index1] = curSum%10;
        }
        for(int j = index1; j > -1; j--){
            int curSum = arr1[j] + remains;
            remains = curSum/10;
            arr1[j] = curSum%10;
            if(remains == 0) break;
        }
        return remains;
    }
}

方法二

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> stack1 = new Stack<>();
        Stack<Integer> stack2 = new Stack<>();
        while(l1 != null){
            stack1.add(l1.val);
            l1 = l1.next;
        }
        while(l2 != null){
            stack2.add(l2.val);
            l2 = l2.next;
        }
        ListNode res = new ListNode(-1);
        int remains = 0;
        while(!stack1.isEmpty() && !stack2.isEmpty()){
            int val = stack1.pop() + stack2.pop() + remains;
            remains = val/10;
            ListNode tmp = new ListNode(val%10);
            tmp.next = res.next;
            res.next = tmp;
        }
        if(!stack1.isEmpty()) handle(stack1,res,remains);
        else handle(stack2,res,remains);
        return res.next;
    }
    private void handle(Stack<Integer> stack, ListNode res, int remains){
        while(!stack.isEmpty()){
            int val = stack.pop() + remains;
            remains = val/10;
            ListNode tmp = new ListNode(val%10);
            tmp.next = res.next;
            res.next = tmp;
        }
        if(remains == 1){
            ListNode tmp = new ListNode(1);
            tmp.next = res.next;
            res.next = tmp;
        }
    }
}