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
        4

    /     

  3         7

          /     

        5         6

For 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：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;
    }
}