59. Spiral Matrix II

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 ]
]

class Solution {
    public int[][] generateMatrix(int n) {
int[][] matrix = new int[n][n];
        int begin = 0, end = n - 1;
        int num = 1;

        while (begin < end) {
            for (int j = begin; j < end; ++j) matrix[begin][j] = num++;
            for (int i = begin; i < end; ++i) matrix[i][end] = num++;
            for (int j = end; j > begin; --j) matrix[end][j] = num++;
            for (int i = end; i > begin; --i) matrix[i][begin] = num++;
            ++begin;
            --end;
        }

        if (begin == end) matrix[begin][begin] = num;

        return matrix;
    }
}

左右

上下

右左

下上

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