59. Spiral Matrix II 螺旋矩阵二

题目链接
tag:

• Medium；

question：
  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 ]
]

思路：
  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，代码如下：

class Solution {
public:
    vector> generateMatrix(int n) {
        vector> res(n, vector(n, 0));
        int up = 0, down = n - 1, left = 0, right = n - 1, val =1;
        while (true) {
            for (int i=left; i<=right; ++i) res[up][i] = val++;
            if (++up > down) break;
            
            for (int i=up; i<=down; ++i) res[i][right] = val++;
            if (--right < left) break;
            
            for (int i=right; i>=left; --i) res[down][i] = val++;
            if (--down < up) break;
            
            for (int i=down; i>=up; --i) res[i][left] = val++;
            if (++left > right) break;
        }
        return res;
    }
};