Minimum number of symbols to change to make a chain

Problem solving with programming: Minimum number of symbols to change to make a chain
http://www.codechef.com/FEB15/problems/CHEFCH

Given a string containing the only symbols '+' or '-', how do we find the minimum number of changes to transform it to a chain. A chain of length N is the sequence of alternative + and - symbols.

For example "+-+-+" is a chain of length 5.

Similarly "-+-+-+" is a chain of length 6.

Input:
2
---+-+-+++
-------
Output:
2
3

Explanation

Example case 1.

We can change symbol 2 from '-' to '+' and symbol 9 from '+' to '-' and receive '-+-+-+-+-+'.

Example case 2.

We can change symbols 2, 4 and 6 from '-' to '+' and receive '-+-+-+-'.

Given the length of the sequence N, there are only two possible chains; One starting with - (-+-+....), and the other starting with + (+-+....).

So it is just enough to compare the input string to these two patterns and find the number of changes to make it a chain. Minimum of these two numbers gives us the answer.

For an example consider the input

+--++

Difference with +-+-+ is 2

Difference with -+-+- is 3

So the minimum number of changes to make it a chain is 2.

 while(t--)


 {


  string input;


  cin >> input;


  int i, result = 0, result1 = 0;


  char p1 = '+', p2 = '-';


  for( i = 0; i < input.size(); i++ )


  {


   if( input[i] != p1 )


    result++;


   if( input[i] != p2 )


    result1++;


   p1 = (p1 == '+')? '-':'+';


   p2 = (p2 == '+')? '-':'+';


  }


  cout << min(result,result1) << endl;


 }

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