Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]

You should return [1,2,3,6,9,8,7,4,5].

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        int m=matrix.size();
        if(m==0) return vector<int>();
        int n=matrix[0].size();
        if(n==0) return vector<int>();
        vector<int>res(m*n);

        int rstart=0,cstart=0;
        int rend=m-1,cend=n-1;
        int index=0;
        
        while(rstart<=rend&&cstart<=cend){
            
            int r=rstart,c=cstart;
            if(cstart==cend){
                while(rstart<=rend) res[index++]=matrix[rstart++][cstart];
                break;
            }
            if(rstart==rend){
                while(cstart<=cend) res[index++]=matrix[rstart][cstart++];
                break;
            }
            while(c<=cend){
                res[index++]=matrix[r][c];
                c++;
            }
            c--;r++;
            while(r<=rend){
                res[index++]=matrix[r][c];
                r++;
            }
            r--;c--;
            while(c>=cstart){
                res[index++]=matrix[r][c];
                c--;
            }
            c++;r--;
            while(r>rstart){
                res[index++]=matrix[r][c];
                r--;
            }
            rstart++;
            cstart++;
            rend--;
            cend--;
        }
        return res;
    }
};