1. 059

spiral-matrix-ii

2. 算法

• O(nm)

3.代码

class Solution {
//key observe: when each component is done, its beginX,endX or beginY,endX will change  
public:
    vector<vector<int> > generateMatrix(int n) {

        if(0 == n) return vector<vector<int> >();
        vector<vector<int> >  result(n, vector<int>(n));
        int beginX = 0, endX = n-1;
        int beginY = 0, endY = n-1;

        int cur = 1;
        int nSquare = n*n;
        while (true) {
            // From left to right  
            for (int i = beginX; i <= endX; ++i)
                result[beginY][i] = cur++;
            if (++beginY > endY) break;
            // From top down  
            for (int i = beginY; i <= endY; ++i)
                result[i][endX] = cur++;
            if (beginX > --endX) break;
            // From right to left  
            for (int i = endX; i >= beginX; --i)
                result[endY][i] = cur++;
            if (beginY > --endY) break;
            // From bottom up  
            for (int i = endY; i >= beginY; --i)
                result[i][beginX] = cur++;
            if (++beginX > endX) break;
        }
        return result;
    }
};