Matrix Zigzag Traversal

题目：

Given a matrix of m x n elements (m rows, n columns),
return all elements of the matrix in ZigZag-order.

Example
Given a matrix:

[
  [1, 2,  3,  4],
  [5, 6,  7,  8],
  [9,10, 11, 12]
]
return [1, 2, 5, 9, 6, 3, 4, 7, 10, 11, 8, 12]

解题思路：
这道题就是用一个方法设置4个边界来loop。

代码：

public int[] printZMatrix(int[][] matrix) {
    if (matrix == null || matrix.length == 0) return null;

    int m = matrix.length - 1, n = matrix[0].length - 1;
    int[] result = new int[(m + 1) * (n + 1)];
    int index = 0;
    for (int i = 0; i <= m + n; i++) {
        int upperBoundx = Math.min(i, m); // x <= m        
        int lowerBoundx = Math.max(0, i - n); // lower bound i - x(y) <= n        
        int upperBoundy = Math.min(i, n); // y <= n        
        int lowerBoundy = Math.max(0, i - m); // i - y(x) <= m        
        if (i % 2 == 0) {
            // column increment            
            for (int y = lowerBoundy; y <= upperBoundy; y++) {
                result[index] = matrix[i - y][y];
                index++;
            }
        } else {
            // row increment            
            for (int x = lowerBoundx; x <= upperBoundx; x++) {
                result[index] = matrix[x][i - x];
                index++;
            }
        }
    }

    return result;
}