CodeForces 628B New Skateboard 思维

B. New Skateboard

time limit per test
1 second

memory limit per test
256 megabytes

input
standard input

output
standard output

Max wants to buy a new skateboard. He has calculated the amount of money that is needed to buy a new skateboard. He left a calculator on the floor and went to ask some money from his parents. Meanwhile his little brother Yusuf came and started to press the keys randomly. Unfortunately Max has forgotten the number which he had calculated. The only thing he knows is that the number is divisible by 4.

You are given a string s consisting of digits (the number on the display of the calculator after Yusuf randomly pressed the keys). Your task is to find the number of substrings which are divisible by 4. A substring can start with a zero.

A substring of a string is a nonempty sequence of consecutive characters.

For example if string s is 124 then we have four substrings that are divisible by 4: 12, 4, 24 and 124. For the string 04 the answer is three: 0, 4, 04.

As input/output can reach huge size it is recommended to use fast input/output methods: for example, prefer to use gets/scanf/printf instead of getline/cin/cout in C++, prefer to use BufferedReader/PrintWriter instead of Scanner/System.out in Java.

Input
The only line contains string s (1 ≤ |s| ≤ 3·105). The string s contains only digits from 0 to 9.

Output
Print integer a — the number of substrings of the string s that are divisible by 4.

Note that the answer can be huge, so you should use 64-bit integer type to store it. In C++ you can use the long long integer type and in Java you can use long integer type.

Examples

input

124

output

4

input

04

output

3

input

5810438174

output

9

题意：给你一串数，让你找出其中有几个子串是4的倍数（包括总串）。

思路：暴搜必定超时O(n^2)，我们可以分类讨论。先考虑一位数，当满足4的倍数时c++；然后是两位数，满足条件+1，这时就要考虑当两位数满足4的倍数时，前面不管再加多少位，都是4的倍数（100、1000、10000...都能被4整除），那么我们就可以把两位数前面的数字（i个）依次加上，他们分别是以两位数为尾的三位数、四位数、五位数...因此c+i+1。O(n+n)。

#include<stdio.h>
#include<string.h>

int main()
{
long long c,i;
char s[300005];
scanf("%s",s);
c=0;
for(i=0;i<strlen(s);i++){
if((s[i]-'0')%4==0) c++;
}
for(i=0;i<strlen(s)-1;i++){
if(((s[i]-'0')*10+(s[i+1]-'0'))%4==0) c+=i+1;
}
printf("%lld\n",c);
return 0;
}