59. Spiral Matrix II

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

循环，创建一个上界和一个下界。
当达到界限时，改变方向。
更新上界和下界的数值。
当上界小于下界时返回。

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] ans = new int[n][n];
        int upperBound = n;
        int lowerBound = 0;
        int i = 0;
        int j = 0;
        int count = 1;
        ans[0][0] = 1;
 
        while ( lowerBound < upperBound ){
            while ( j < upperBound-1 ){
                j++;
                count++; 
                ans[i][j] = count;           
            }
            while ( i < upperBound-1 ){
                i++;
                count++;
                ans[i][j] = count;
            }
            while ( j > lowerBound ){
                j--;
                count++;
                ans[i][j] = count;
            }
            upperBound--;
            lowerBound++;
            while ( i > lowerBound ){
                i--;
                count++;
                ans[i][j] = count;
            }
        }
        return ans;
    }
}