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
/ \
/ \
For and , the LCA is .
For and , the LCA is .
For and , the LCA is .
更复杂的参考：http://www.cnblogs.com/EdwardLiu/p/4265448.html
public class Solution {
/**
* @param root: The root of the binary search tree.
* @param A and B: two nodes in a Binary.
* @return: Return the least common ancestor(LCA) of the two nodes.
*/
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode A, TreeNode B) {
// write your code here
if (root == null) return null;
if (root==A || root==B) return root;
TreeNode lch = lowestCommonAncestor(root.left, A, B);
TreeNode rch = lowestCommonAncestor(root.right, A, B);
if (lch!=null && rch!=null) return root;
return lch==null? rch : lch;
}
}