Lowest Common Ancestor

Note:

// 在root为根的二叉树中找A,B的LCA:
// 如果找到了就返回这个LCA
// 如果只碰到A，就返回A
// 如果只碰到B，就返回B
// 如果都没有，就返回null

Just consider relationship between left, right and root. If both left and right are found, LCA must be the root. If only left is found, this means two nodes are both in the left side, so LCA should be left. If only right is found, this means two nodes are both in right side, LCA should be the right. 

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
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
        return findLCA(root, A, B);
    }
    
    private TreeNode findLCA(TreeNode root, TreeNode A, TreeNode B) {
        if (root == null || root == A || root == B) {
            return root;
        }
        
        TreeNode left = findLCA(root.left, A, B);
        TreeNode right = findLCA(root.right, A, B);
        
        if (left != null && right != null) {
            return root;
        }
        
        if (left != null) {
            return left;
        }
        
        if (right != null) {
            return right;
        }
        
        return null;
    }
}