[LeetCode] Spiral Matrix II 螺旋矩阵之二

Given a positive integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

Example:

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

 

此题跟之前那道 Spiral Matrix 本质上没什么区别，就相当于个类似逆运算的过程，这道题是要按螺旋的顺序来填数，由于给定矩形是个正方形，我们计算环数时用 n / 2 来计算，若n为奇数时，此时最中间的那个点没有被算在环数里，所以最后需要单独赋值，还是下标转换问题是难点，参考之前 Spiral Matrix 的讲解来转换下标吧，参见代码如下：

解法一：

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> res(n, vector<int>(n));
        int val = , p = n;
        for (int i = ; i < n / ; ++i, p -= ) {
            for (int col = i; col < i + p; ++col)
                res[i][col] = val++;
            for (int row = i + ; row < i + p; ++row)
                res[row][i + p - ] = val++;
            for (int col = i + p - ; col >= i; --col)
                res[i + p - ][col] = val++;
            for (int row = i + p - ; row > i; --row)
                res[row][i] = val++;
        }
        if (n %  != ) res[n / ][n / ] = val;
        return res;
    }
};

当然我们也可以使用下面这种简化了坐标转换的方法，博主个人还是比较推崇下面这种解法，不容易出错，而且好理解，参见代码如下：

解法二：

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> res(n, vector<int>(n));
        int up = , down = n - , left = , right = n - , val = ;
        while (true) {
            for (int j = left; j <= right; ++j) res[up][j] = val++;
            if (++up > down) break;
            for (int i = up; i <= down; ++i) res[i][right] = val++;
            if (--right < left) break;
            for (int j = right; j >= left; --j) res[down][j] = val++;
            if (--down < up) break;
            for (int i = down; i >= up; --i) res[i][left] = val++;
            if (++left > right) break;
        }
        return res;
    }
};

Github 同步地址：

https://github.com/grandyang/leetcode/issues/59

类似题目：

Spiral Matrix

参考资料：

https://leetcode.com/problems/spiral-matrix-ii/

https://leetcode.com/problems/spiral-matrix-ii/discuss/22392/C%2B%2B-template-for-Spiral-Matrix-and-Spiral-Matrix-II

https://leetcode.com/problems/spiral-matrix-ii/discuss/22289/My-Super-Simple-Solution.-Can-be-used-for-both-Spiral-Matrix-I-and-II

LeetCode All in One 题目讲解汇总(持续更新中...)