func findDiagonalOrder(matrix [][]int) (ans []int) {
	m := len(matrix)
	if m == 0 {
		return
	}
	n := len(matrix[0])
	if n == 0 {
		return
	}
	x, y := 0, 0
	dx, dy := 1, -1
	for i := 0; i < m*n; i++ {
		ans = append(ans, matrix[y][x])
		if ny := y + dy; (ny == -1 || ny == m) && x != n-1 {
			dx, dy = -dx, -dy
			x++
		} else if nx := x + dx; nx == -1 || nx == n {
			dx, dy = -dx, -dy
			y++
		} else {
			x, y = nx, ny
		}
	}
	return
}