# (LOT95) You are given a tree of n <= 1000000 nodes and n - 1 edges. Each node of the tree contains a light bulb and a button. Pressing the button in a node switches the state of the light bulb and the adjacent light bulbs. Come up with an algorithm that finds out the *minimum* number of button presses to switch all the light bulbs “off”
# (SGU) You are given an undirected graph of n <= 100 nodes. Each node contains a light bulb and a button. Pressing the button in a node toggles the state of the light bulb and its adjacent light bulbs. Come up with an algorithm that finds out *if there is* any solution that turns all the lights “off”.
# (SGU/TOPCODER) Same setup as 6. Come up with an algorithm that finds out *how many* different solutions there are.

# (LOT06) You are given a grid of lights of size NxM (N, M <= 1000). Some lights are “on”, some are “off”. A move consists in choosing a cell and switching the state of the lights on the same row and same column but leaving the chosen cell in the same state. Come up with an algorithm that finds out the *minimum* number of moves to turn all the lights “off”.

# (LOT06) You are given a grid of lights of size NxM (N, M <= 1000). Some lights are “on”, some are “off”. A move consists in choosing a cell and switching the state of the lights on the same row and same column but leaving the chosen cell in the same state. Come up with an algorithm that finds out the *minimum* number of moves to turn all the lights “off”.
# (CEOI 2000) 'Planet-X':http://ceoi.inf.elte.hu/probarch/00/p1.htm