Add more efficient solution to lowest common ancestor

[idf]

Location: Binary tree page 10. Current solution is inefficient. Need to add more efficient algorithm.

[idf]

Similar to https://www.geeksforgeeks.org/lowest-common-ancestor-binary-tree-set-1/ but the explanation of it is weak. But it returns is not direct LCA - what the recursive function returns is actually the subtree root that containing the at least one of the nodes.