498. Diagonal Traverse

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Given a matrix of M x N elements (M rows, N columns), return all elements of the matrix in diagonal order as shown in the below image.

Example:

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Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output:  [1,2,4,7,5,3,6,8,9]
Explanation:

img

Note:
The total number of elements of the given matrix will not exceed 10,000.

解法1：

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class Solution {
    public int[] findDiagonalOrder(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0) {
            return new int[0];
        }
        int m = matrix.length;
        int n = matrix[0].length;
        int[] res = new int[m * n];
        int[][] dirs = new int[][]{{-1,1},{1,-1}};
        int d = 0;
        int row = 0, col = 0;
        for (int i = 0; i < m * n; i++) {
            res[i] = matrix[row][col];
            row += dirs[d][0];
            col += dirs[d][1];
            if (row >= m) {
                row = m - 1;
                col += 2;
                d = 1 - d;
            }
            if (col >= n) {
                col = n - 1;
                row += 2;
                d = 1 - d;
            }
            if (row < 0) {
                row = 0;
                d = 1 - d;
            }
            if (col < 0) {
                col = 0;
                d = 1 - d;
            }
        }
        return res;
    }
}