Given the root and two nodes in a Binary Tree. Find the lowest common ancestor(LCA) of the two nodes.The lowest common ancestor is the node with largest depth which is the ancestor of both nodes.Example        4    /     \  3         7          /     \        5         6For 3 and 5, the LCA is 4.For 5 and 6, the LCA is 7.For 6 and 7, the LCA is 7.

更复杂的参考：

1 public class Solution { 2     /** 3      * @param root: The root of the binary search tree. 4      * @param A and B: two nodes in a Binary. 5      * @return: Return the least common ancestor(LCA) of the two nodes. 6      */ 7     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode A, TreeNode B) { 8         // write your code here 9         if (root == null) return null;10         if (root==A || root==B) return root;11         TreeNode lch = lowestCommonAncestor(root.left, A, B);12         TreeNode rch = lowestCommonAncestor(root.right, A, B);13         if (lch!=null && rch!=null) return root;14         return lch==null? rch : lch;15     }16 }