log.Println(solveSecond(m))

func solveSecond(m matrix) int {
	var step int
	for ; ; step++ {
		for row := range m {
			for col := range m[0] {
				m[row][col]++
			}
		}
		s := seen{}
		for row := range m {
			for col := range m[0] {
				s.flash(m, row, col)
			}
		}
		s = seen{} // reset flashes
		for row := range m {
			for col := range m[0] {
				if m[row][col] > 9 {
					m[row][col] = 0
				}
			}
		}
		if m.fullFlash() {
			break
		}
	}
	return step + 1
}
func (m matrix) fullFlash() bool {
	for row := range m {
		for col := range m[0] {
			if m[row][col] != 0 {
				return false
			}
		}
	}
	return true
}