26-树的子结构

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

class Solution {
    public boolean isSubStructure(TreeNode A, TreeNode B) {
        boolean result= false;
        if (A != null && B!= null) {
            if (A.val == B.val) result = hasSubTree(A, B);
            if (!result) result = isSubStructure(A.left, B);
            if (!result) result = isSubStructure(A.right, B);
        }
        return result;
    }

    public boolean hasSubTree(TreeNode a, TreeNode b) {
        // b 为空说明子树已经全部遍历完了
        if (b == null) {
            return true;
        } else if (a == null) {
            return false;
        }
        
        if (a.val != b.val) return false;
        return hasSubTree(a.left, b.left) && hasSubTree(a.right, b.right);
    }
}