树的子结构

输入两棵二叉树A，B，判断B是不是A的子结构。（ps：我们约定空树不是任意一个树的子结构）

package 剑指Offer.树的子结构;

public class Solution {
    public boolean HasSubtree(TreeNode root1, TreeNode root2) {
        if (root2 == null) {
            return false;
        }

        if (root1 == null) {
            return false;
        }

        boolean flag = false;

        if (root1.val == root2.val) {
            flag = isSubtree(root1.left, root2.left) && isSubtree(root1.right, root2.right);
        }

        if (!flag) {
            flag = HasSubtree(root1.left, root2);

            if (!flag) {
                flag = HasSubtree(root1.right, root2);
            }
        }

        return flag;
    }

    private boolean isSubtree(TreeNode root1, TreeNode root2) {
        if (root2 == null) {
            return true;
        }

        if (root1 == null) {
            return false;
        }

        if (root1.val == root2.val) {
            return isSubtree(root1.left, root2.left) && isSubtree(root1.right, root2.right);
        }

        return false;
    }
}

class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}