Smallest Subtree with all the Deepest Nodes 包含最深结点的最小子树
Givenabinarytreerootedatroot,thedepthofeachnodeistheshortestdistancetotheroot.Anodeisdeepestifithasthelargestdepthpossibleamonganynodeintheentiretree.Thesubtreeofanodeisthatnode,plusthesetofalldescend