http://www.lintcode.com/en/problem/subtree/

1. have a helper function to identify whether the two are identical. and divide and conquer on the large tree until there is a match of T1. currentNode.val = T2.root.val and call the helper function
2. since it is sub tree that means all the way to the leaf node on both.

/**
 * 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 T1, T2: The roots of binary tree.
     * @return: True if T2 is a subtree of T1, or false.
     */
    public boolean isSubtree(TreeNode T1, TreeNode T2) {
        // write your code here
        if (T1 == null && T2 == null) {
            return true;
        }

        if (T2 == null) {
            return true;
        }

        if (T1 == null) {
            return false;
        }

        if (T1.val == T2.val) {
            if (isIdentical(T1, T2)) {
                return true;
            }
        }

        boolean left = isSubtree(T1.left, T2);
        boolean right = isSubtree(T1.right, T2);

        return left || right;



    }

    public boolean isIdentical(TreeNode T1, TreeNode T2) {
        if (T1 == null && T2 == null) {
            return true;
        }

        if (T1 == null || T2 == null) {
            return false;
        }


        boolean left = isIdentical(T1.left, T2.left);
        boolean right = isIdentical(T1.right, T2.right);

        return T1.val == T2.val && left && right;


    }
}

1. traverse method

public class Solution {
    /**
     * @param T1, T2: The roots of binary tree.
     * @return: True if T2 is a subtree of T1, or false.
     */
    public boolean isSubtree(TreeNode T1, TreeNode T2) {
        // write your code here
        if (T2 == null) {
            return true;
        }

        if (T1 == null) {
            return false;
        }

        if (T1.val == T2.val && isIdentical(T1, T2)) {
            return true;
        }

        if (isSubtree(T1.left, T2) || isSubtree(T1.right, T2)) {
            return true;
        }

        return false;

    }

    public boolean isIdentical(TreeNode T1, TreeNode T2) {
        if (T1 == null && T2 == null) {
            return true;
        }

        if (T1 == null || T2 == null) {
            return false;
        }

        return T1.val == T2.val && isIdentical(T1.left, T2.left) && isIdentical(T1.right, T2.right);
    }
}