59.螺旋矩阵 II

解题思路

初看，毫无头绪。
打开官方答案领会一下。

整体采用构建矩阵，填充矩阵的思路，填充过程分为四种情况：

1. 从左到右填充一行
2. 从上到下填充一列
3. 从右到左填充一行，注意只有一行的情况
4. 从下到上填充一列，注意只有一列的情况

出现的问题

JavaScript解法代码

var generateMatrix = function(n) {
    let num = 1;
    const matrix = new Array(n).fill(0).map(() => new Array(n).fill(0));
    let left = 0, right = n - 1, top = 0, bottom = n - 1;
   while (left <= right && top <= bottom) {
        // 从左上至右上
        for (let column = left; column <= right; column++) {
            matrix[top][column] = num;
            num++;
        }
        // 从右上至右下
        for (let row = top + 1; row <= bottom; row++) {
            matrix[row][right] = num;
            num++;
        }
        // 还没到最后一圈
        if (left < right && top < bottom) {
            // 从右下至左下
            for (let column = right - 1; column > left; column--) {
                matrix[bottom][column] = num;
                num++;
            }
            // 从左下至左上
            for (let row = bottom; row > top; row--) {
                matrix[row][left] = num;
                num++;
            }
        }
        left++;
        right--;
        top++;
        bottom--;
    }
    return matrix;
};

Python解法代码

import numpy as np
class Solution(object):
    def generateMatrix(self, n):
        """
        :type n: int
        :rtype: List[List[int]]
        """
        matrix = np.zeros((n, n))
        num = 1
        left=0
        right=n-1
        top=0
        bottom=n-1
        while(left <= right and top <= bottom):
            for i in range(left,right+1):
                matrix[top][i] = num
                num += 1
            for j in range(top+1,bottom+1):
                matrix[j][right] = num
                num += 1
            if(left < right and top < bottom):
                for i in range(right-1,left,-1):
                    matrix[bottom][i] = num
                    num += 1
                for j in range(bottom,top,-1):
                    matrix[j][left] = num
                    num += 1
            left += 1
            right -= 1
            top += 1
            bottom -= 1

        # convert the matrix to integer type and then to a list
        matrix = matrix.astype(int).tolist()
        return matrix
        

总结：

1. 注意数组的初始化：
   nums = [[0] * n for _ in range(n)] # 创建一个大小为n的二维数组，初始值都为0
const matrix = new Array(n).fill(0).map(() => new Array(n).fill(0));
matrix = np.zeros((n, n))