Skill tree rules
You can buy a node when one of the nodes linked before it is bought. One is enough. Some nodes come in locked groups: buying one closes the others.
The rules
- Your starting nodes are open from the start and cost nothing.
- Any other node opens when at least one of its parent nodes has the points it asks for.
- Links run one way. A bought child does not open its parent.
- A node locked with a bought node cannot be bought until you refund that one.
- Every node takes 1 point.
Refunds
Starting nodes cannot be refunded. A node that a bought node depends on cannot be refunded first. The game says:
Refund the nodes that depend on this one first
Numbers
Nodes in all trees2496
Nodes with more than one parent997
Locked pairs427
Examples
Power Hook has 3 parents. Any one of them opens it: 
Blunt Damage Increase, 
Attack Speed Increase, 
Attack Damage Increase.
Quick Strikes and 
Cycling Strikes are locked together. Buy one and the other stays closed.