Rotate a matrix by 90 degrees in place: http://leetcode.com/onlinejudge#question_48

rotate_matrix.cpp

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        int N = matrix.size()-1,
            temp = 0;
        
        for (int i=0; i < N; i++) {
            for (int j=0; j+i < N; j++) {
                
                //Only one temp variable is needed             
                temp = matrix[i][N-j]; 
                
                //r is constant, col moves back
                matrix[i][N-j] = matrix[i+j][i];
                
                //row moves down, col stays constant
                matrix[i+j][i] = matrix[N][i+j];
                
                //col moves forward, row stays constant
                matrix[N][i+j] = matrix[N-j][N];
                
                //row moves up, col stays constant
                matrix[N-j][N] = temp;
            }
            N--;
        }
    }
};

Good one! I used a bunch of variables to do this but your solution is much cleaner.