59. Spiral Matrix II

螺旋矩阵

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:

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

思路：

重复项为一圈一圈的数值 

java:

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] res = new int[n][n];
        if(n==0) return res;
        int row_start = 0, row_end = n-1;
        int col_start = 0, col_end = n-1;
        int count = 1;
        
        while(row_start<=row_end&&col_start<=col_end){
            
            for(int i=col_start;i<=col_end;i++){
                res[row_start][i] = count++;
            }
            row_start++;
            
            for(int i=row_start;i<=row_end;i++){
                res[i][col_end] = count++;
            }
            col_end--;
            
            for(int i=col_end;i>=col_start;i--){
                if(row_start<=row_end)
                    res[row_end][i] = count++;
            }
            row_end--;
            
            for(int i=row_end;i>=row_start;i--){
                if(col_start<=col_end)
                    res[i][col_start] = count++;
            }
            col_start++;
            
        }
        return res;
    }
}