题目：
You are given a rectangular grid with n rows and m columns. The cell located on the i-th row from the top and the j-th column from the left has a value aij written in it.
You can perform the following operation any number of times (possibly zero):
Choose any two adjacent cells and multiply the values in them by −1. Two cells are called adjacent if they share a side.
Note that you can use a cell more than once in different operations.
You are interested in X, the sum of all the numbers in the grid.
What is the maximum X you can achieve with these operations?
Input
Each test contains multiple test cases. The first line contains the number of test cases t (1≤t≤100). Description of the test cases follows.
The first line of each test case contains two integers n,m (2≤n, m≤10).
The following n lines contain m integers each, the j-th element in the i-th line is aij (−100≤aij≤100).
Output
For each testcase, print one integer X, the maximum possible sum of all the values in the grid after applying the operation as many times as you want.
Example
inputCopy
2
2 2
-1 1
1 1
3 4
0 -1 -2 -3
-1 -2 -3 -4
-2 -3 -4 -5
outputCopy
2
30
Note
In the first test case, there will always be at least one −1, so the answer is 2.
In the second test case, we can use the operation six times to elements adjacent horizontally and get all numbers to be non-negative. So the answer is: 2×1+3×2+3×3+2×4+1×5=30.
解题思路：
如果里面有0的话，所有的负数都可以变成正数，
如果负数的个数为偶数的话，所有负数都可以变成正数
如果负数个数为奇数，不能使所有的负数变成正数，必然会有一个为负，现在只需要将绝对值最小的那个数变为负数就好。
上代码！
#include <bits/stdc++.h>
using namespace std;
int a[200][200];
int main()
{
int t;
cin>>t;
while(t--){
int n,m;
int count=0,ret=0;
long long sum=0;
int mi=101;
scanf("%d %d",&n,&m);
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++){
scanf("%d",&a[i][j]);
if(a[i][j]<0) count++;
if(a[i][j]==0) ret=1;
if(abs(a[i][j])<mi){
mi=abs(a[i][j]);
}
sum+=abs(a[i][j]);
}
}
if(count%2!=0&&ret!=1){
sum=sum-2*mi;
}
cout<<sum<<endl;
}
return 0;
}
来源：oschina
链接：https://my.oschina.net/u/4277087/blog/4725176