ACM集训STL(1)A题

A - Most Unstable Array

You are given two integers n and m. You have to construct the array a of length n consisting of non-negative integers (i.e. integers greater than or equal to zero) such that the sum of elements of this array is exactly m and the value ∑i=1n−1|ai−ai+1| is the maximum possible. Recall that |x| is the absolute value of x.

In other words, you have to maximize the sum of absolute differences between adjacent (consecutive) elements. For example, if the array a=[1,3,2,5,5,0] then the value above for this array is |1−3|+|3−2|+|2−5|+|5−5|+|5−0|=2+1+3+0+5=11. Note that this example doesn't show the optimal answer but it shows how the required value for some array is calculated.

You have to answer t independent test cases.

Input

The first line of the input contains one integer t (1≤t≤104) — the number of test cases. Then t test cases follow.

The only line of the test case contains two integers n and m (1≤n,m≤109) — the length of the array and its sum correspondingly.

Output

For each test case, print the answer — the maximum possible value of ∑i=1n−1|ai−ai+1| for the array a consisting of n non-negative integers with the sum m.

题解

#include<iostream>
using namespace std;
int main()
{
    int m,n,T;
    cin>>T;
    while(T--)
    {
        cin>>m>>n;
        if(m==1){cout<<0<<endl;continue;}
        if(m==2){cout<<n<<endl;continue;}
        if(m>2){cout<<2*n<<endl;continue;}
    }
    return 0;
}