Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order. For example, Given n = 3, You should return the following matrix: [ [ 1, 2, 3 ], [ 8, 9, 4 ], [ 7, 6, 5 ] ] http://leetcode.com/onlinejudge#question_59

leetcode_spirial_matrix_2.cpp

int dir[4][2] = {{0,1},{1,0},{0,-1},{-1,0}};

bool inBound(int x, int y, int n) {
    return x >= 0 && x < n && y >= 0 && y < n;
}

class Solution {
public:
    vector<vector<int> > generateMatrix(int n) {
        vector<vector<int> > ans;
        if (n == 0)
            return ans;
        ans.resize(n);
        for (int i = 0; i < n; ++i) {
            ans[i].resize(n);
            for (int j = 0; j < n; ++j)
                ans[i][j] = 0;
        }
        int x = 0, y = 0;
        int d = 0;
        ans[x][y] = 1;
        for (int val = 2; val <= n * n; val++) {
            while (true) {
                int nx = x + dir[d][0], ny = y + dir[d][1];
                if (inBound(nx, ny, n) && ans[nx][ny] == 0) {
                    ans[nx][ny] = val;
                    x = nx;
                    y = ny;
                    break;
                }
                d = (d + 1) % 4;
            }
        }
        return ans;
    }
};