CTCI 4.7

/* use find() to determin whether the tree contains the two targets, if contains, update the result. Then try
to determine whether the two subtrees contains the targets. Since the first common ancestor would be the last
node find to contain the targets, the latest result would just be what we want.
*/
public class FirstCommonAncestor {
    TreeNode res = null;
    public void fircomanc(TreeNode node, TreeNode tarA, TreeNode tarB) {
        if(find(node, tarA) == true && find(node, tarB) == true) res = node;
        if(node != null) {
            fircomanc(node.left, tarA, tarB);
            fircomanc(node.right, tarA, tarB);
        }
    } 

    private boolean find(TreeNode node, TreeNode tar) {
        if(node == tar) return true;

        if(node != null) return (find(node.left, tar) | find(node.right, tar));
        return false;
    }

    public static void main(String[] args) {
         TreeNode node1 = new TreeNode(1);
         TreeNode node2 = new TreeNode(2);
         TreeNode node3 = new TreeNode(3);
         TreeNode node4 = new TreeNode(4);
         TreeNode node5 = new TreeNode(5);
         TreeNode node6 = new TreeNode(6);
         TreeNode node7 = new TreeNode(7);
         node5.left = node3; node5.right = node6;
         node3.left = node1; node3.right = node4;
         node1.right = node2;
         FirstCommonAncestor fca = new FirstCommonAncestor();
         fca.fircomanc(node5, node2, node2);
         if(fca.res == null) System.out.println("No Common Ancestor");
         else
             System.out.println(fca.res.val);
     }
}