Spiral Matrix II. Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

spiral_matrix_II.cpp

class Solution {
public:
    vector<vector<int> > generateMatrix(int n) {
        vector<vector<int> > vvInt(n, vector<int>(n,0));
        
        //fill two strips consecutively,
        //      First row,  last col, last row, first col
        //Note that if n is odd (n >=3), the central one is NOT filled (it's not in four-strip structure)
        int rowStart = 0, colStart = 0;
        int scan = 1;
        while (rowStart < n) {
            // first row [row, col].... [row, n-1-colStart-1]
            int row=rowStart, col=colStart;
            for(; col<=n-1-colStart-1; col++) {
                vvInt[row][col] = scan ++;
            }
            col = n-1-colStart; //set for sure
            //last col, [row, n-1-colStart] ... [n-1-rowStart-1, n-1-colStart]
            for(; row<=n-1-rowStart-1; row++) {
                vvInt[row][col] = scan ++;
            }
            row = n-1-rowStart;
            //last row, [n-1-rowStart, n-1-colStart] -- [n-1-rowStart, colStart+1]
            for(; col >= colStart+1; col--) {
                vvInt[row][col] = scan ++;
            }
            
            col = colStart;
            // first col, [n-1-rowStart, colStart] ... [rowStart, colStart]
            for(; row>=rowStart+1; row --)  {
                vvInt[row][col] = scan ++;
            }
            
            //this circle done; go inside
            rowStart ++;
            colStart ++;
        }
        
        // above it's assumed matrix has four-strips, so when n is odd, the centeral one is NOT filled
        if (n %2 )
            vvInt[n/2][n/2] = scan;
        return vvInt;
    }
};