Binary Prefix Divisible By 5

https://leetcode.com/contest/weekly-contest-130/problems/binary-prefix-divisible-by-5/

Given an array A of 0s and 1s, consider N_i: the i-th subarray from A[0] to A[i] interpreted as a binary number (from most-significant-bit to least-significant-bit.)

Return a list of booleans answer, where answer[i] is true if and only if N_i is divisible by 5.

Example 1:

Input: [0,1,1]

Output: [true,false,false]

Explanation:

The input numbers in binary are 0, 01, 011; which are 0, 1, and 3 in base-10. Only the first number is divisible by 5, so answer[0] is true.

Example 2:

Input: [1,1,1]

Output: [false,false,false]

Example 3:

Input: [0,1,1,1,1,1]

Output: [true,false,false,false,true,false]

Example 4:

Input: [1,1,1,0,1]

Output: [false,false,false,false,false]

Solution

每增加1位，0是乘二，1是乘2+1.

class Solution {
public:
    vector<bool> prefixesDivBy5(vector<int>& A) {
        vector<bool> res;
        int v = 0;
        for (int a : A) {
            v *= 2;
            v += (a == 1 ? 1 : 0);
            v %= 10;
            res.push_back(v % 5 == 0);
        }
        return res;
    }
};