solved 'Spiral Matrix II ' on LeetCode http://oj.leetcode.com/problems/spiral-matrix-ii/

spiral_matrix_ii.cpp

class Solution {
public:
    enum Direction {RIGHT, LEFT, DOWN, UP};
    vector<vector<int> > generateMatrix(int n) {
        // init 
        i = 0; j = 0; matrix_size = n;
        result.clear();
        vector<int> v;
        Direction d = RIGHT;
        for (int i=0; i<n; ++i) v.push_back(0);
        for (int i=0; i<n; ++i) result.push_back(v);
        
        for (int ii=0; ii<n*n; ++ii)
            d = mark(d, ii+1);
        
        return result;
    }
private:
    int i, j, matrix_size;
    vector<vector<int> > result;
    
    Direction mark(const Direction d, int m) {
        result[i][j] = m;
        Direction next_d = d;
        switch (d) {
            case RIGHT:
                if (j == matrix_size-1 ||
                        result[i][j+1] != 0) {
                    next_d = DOWN;
                    ++i;
                }
                else 
                    ++j;
                break;
            case DOWN:
                if (i == matrix_size-1 ||
                        result[i+1][j] != 0) {
                    next_d = LEFT;
                    --j;
                }
                else 
                    ++i;
                break;
            case LEFT:
                if (j == 0 || result[i][j-1] != 0) {
                    next_d = UP;
                    --i;
                }
                else 
                    --j;
                break;
            case UP:
                if (result[i-1][j] != 0) {
                    next_d = RIGHT;
                    ++j;
                }
                else 
                    --i;
        }
        return next_d;
    }
};