Lintcode: Lowest Common Ancestor

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;
     }
 }