498. Diagonal Traverse

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:

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

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

Explanation:

Note:

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

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

int row = 0, col = 0, pos = 0, m = matrix.length, n=matrix[0].length, output [] = new int[m * n];

for (pos = 0; pos < m * n; pos++) {
    output[pos] = matrix[row][col];

    if ((row + col) % 2 == 0) {
        // The direction is always up when the sum of row & col is even
        
        // For last column, go down
        if (col == n-1) { row++; }                
        
        // For first row & non-last columns, go right
        else if (row == 0) { col++; }
        
        // For not first row & non-last columns, go up and to the right
        else { row--; col++; }

    } else {
        // The direction is always down when the sum of row & col is odd

        // For last row, go right
        if (row == m-1) { col++; } 
        
        //  For non-last row & first column, go down
        else if (col == 0) { row++; }
        
        // For non-last row & non-first column, go down and to the left
        else { row++; col--; }
    }
}

https://leetcode.com/problems/diagonal-traverse/discuss/97711/Java-15-lines-without-using-boolean

通过观察发现 i + j % 2 == 0时是往右上走，其中有两个边界条件，1.到最右列时要往下走，2. 在第一行时往右， 3.一般情况，向右上走

i + j % 2 ！= 0时往坐下走，也有两个边界条件，1. 到最后一行时往右走，2. 在第一列时往下走，3.一般情况，往左下走

下面还有为什么先判断行/列边界条件，而且if else语句顺序不能变

you will get IndexOutBound Error. The reason is following:
there will be a situation when we satisfy both these two statements, in case i, it is at right top corner, in this case, we can only goes down -- thus it has to first goto "col == n-1" check. Otherwise if goes to "row == 0" check we will have indexOutOfBound error since we can't do col++;

Similarly, there will be a situation when we satisfy both these two statements, in case ii, it is at left bottom corner, in this case, we can only goes right --it has to first goto "row == m-1" check, Otherwise if goes to "col == 0" check we will have indexOutOfBound error since we can't do row++;