3
    / \
   4   5
  / \
 1   2

   4 
  /
 1

输入：A = [1,2,3], B = [3,1]
输出：false

输入：A = [3,4,5,1,2], B = [4,1]
输出：true

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSubStructure(TreeNode A, TreeNode B) {
        return (A!=null && B!=null) &&(doesAHaveB(A,B)||isSubStructure(A.left,B)||isSubStructure(A.right,B));
    }
    //判断B是否是A的子树 
    //先判断根节点是否相同 如果不同直接返回false
    //如果相同再递归判断左右子树
    public boolean doesAHaveB(TreeNode A,TreeNode B){
        if(B == null){
            return true;
        }
        if(A == null){
            return false;
        }
        if(A.val != B.val){
            return false;
        }
        return doesAHaveB(A.left,B.left) && doesAHaveB(A.right,B.right);
    }
}