Binary Tree Level Order Traversal——Breadth-first Search
Givenabinarytree,returnthelevelordertraversalofitsnodes'values.(ie,fromlefttoright,levelbylevel).Forexample:Givenbinarytree[3,9,20,null,null,15,7],3/\920/\157returnitslevelordertraversalas:[[3],[9,20]