剑指 Offer 29. 顺时针打印矩阵

class Solution {
    
    public int[] spiralOrder(int[][] matrix) {
    
        int m = matrix.length;        
        if (m == 0) return new int[]{
    }; 
        int n = matrix[0].length;  
        int[] res = new int[m * n];
        int up = 0, down = m - 1;       
        int left = 0, right = n - 1;  
        int idx = 0;  
        while (left <= right && up <= down) {
               
            for (int i = left; i <= right; i++) {
    
                res[idx++] = matrix[up][i];
            }
            up++;   
           
            for (int i = up; i <= down; i++) {
    
                res[idx++] = matrix[i][right];
            }
            right--;  

            if (left > right || up > down) break;
           
            for (int i = right; i >= left; i--) {
    
                res[idx++] = matrix[down][i];
            }
            down--;        
            
            for (int i = down; i >= up; i--) {
    
                res[idx++] = matrix[i][left];
            }
            left++;  
        }
        return res;
    }
}