59 Spiral Matrix II

Given an integern, generate a square matrix filled with elements from 1 ton2in spiral order.

For example,
Givenn=3,

You should return the following matrix:

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

Solution)

class Solution {
    public int[][] generateMatrix(int n) {
        if (n == 0) return new int[0][0];
        int[][] result = new int[n][n];
        int count = 1;
        int step = 0;
        while(count < n*n) {
            int i = step;
            int j = step;
            for (; j < n-step-1; j++) result[i][j] = count++;
            for (; i < n-step-1; i++) result[i][j] = count++;
            for (; j > step; j--) result[i][j] = count++;
            for (; i > step; i--) result[i][j] = count++;
            step++;
        }
        if (result[step][step] == 0)
            result[step][step] = count;
        return result;
    }
}

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] ret = new int[n][n];
        int left = -1,top = 0;
        int right = n -1,down = n - 1;
        int count = 1;
        while (left <= right) {
            for (int j = ++left; j <= right; j++) ret[top][j] = count++;
            for (int i = ++top; i <= down; i++) ret[i][right] = count++;
            for (int j = --right; j >= left; j--) ret[down][j] = count++;
            for (int i = --down; i >= top; i--) ret[i][left] = count++;
        }
        return ret;
    }
}

class Solution {
    public int[][] generateMatrix(int n) {
       if (n==0) return new int[n][n];
       int[] dc = new int[]{1,0,-1,0};
       int[] dr = new int[]{0,1,0,-1};
       int dir = 0, val=0, r=0, c=0,limit=n*n;
       int[][] matrix = new int[n][n];
       while (val++ < limit) {
        matrix[r][c] = val;
        r += dr[dir];
        c += dc[dir];
        if (isInvalid(matrix,r, c)) {
         r-= dr[dir];
         c-=dc[dir];
         dir = (dir+1)%4;
         r+= dr[dir];
         c+= dc[dir];
        }
       }
       return matrix;
      }
      private static boolean isInvalid(int[][] m, int r, int c) {
       return r<0||c<0||r>=m.length||c>= m.length||m[r][c] != 0;
      } 
}