[leetcode #1290] Convert Binary Number in a Linked List to Integer

Problem

Given head which is a reference node to a singly-linked list. The value of each node in the linked list is either 0 or 1. The linked list holds the binary representation of a number.
Return the decimal value of the number in the linked list.
Example 1:

Input: head = [1,0,1]
Output: 5
Explanation: (101) in base 2 = (5) in base 10

Example 2:

Input: head = [0]
Output: 0

Example 3:

Input: head = [1]
Output: 1

Example 4:

Input: head = [1,0,0,1,0,0,1,1,1,0,0,0,0,0,0]
Output: 18880

Example 5:

Input: head = [0,0]
Output: 0

Constraints:

・ The Linked List is not empty.
・ Number of nodes will not exceed 30.
・ Each node's value is either 0 or 1.

Idea

本当に簡単な問題ですね.これは,リストの周りにノード順にビットアレイを求め,そのビット値がどれだけあるかを求める問題である.
ビットの特徴は、各ビット数が2の倍数であることです.ノードの周囲で返される値を2倍に変更した後、ノードの値が1の場合は1を追加します.この手順を繰り返すと、値を返すだけです.

Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public int getDecimalValue(ListNode head) {
        int res = 0;
        ListNode node = head;

        while (node != null) {
            res = 2 * res + node.val;
            node = node.next;
        }

        return res;
    }
}

Reference

https://leetcode.com/problems/convert-binary-number-in-a-linked-list-to-integer/