Link nodes at same level in a binary tree

Tests:

Start testing tree with nodes: 1000000 time: 30 memory: 32347072

Start testing tree with nodes: 2000000 time: 27 memory: 64337088

Start testing tree with nodes: 3000000 time: 50 memory: 96336888

Start testing tree with nodes: 4000000 time: 78 memory: 128336888

Start testing tree with nodes: 5000000 time: 114 memory: 160336888

Start testing tree with nodes: 6000000 time: 119 memory: 192336888

Start testing tree with nodes: 7000000 time: 167 memory: 224336888

Start testing tree with nodes: 8000000 time: 173 memory: 256336888

Start testing tree with nodes: 9000000 time: 280 memory: 288336888