Example 1:
Given tree s:
​
     3
    / \
   4   5
  / \
 1   2
Given tree t:
   4 
  / \
 1   2
Return true, because t has the same structure and node values with a subtree of s.
Example 2:
Given tree s:
​
     3
    / \
   4   5
  / \
 1   2
    /
   0
Given tree t:
   4
  / \
 1   2
Return false.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
bool isEqual(TreeNode*s,TreeNode*t){
    if(!t) return s? false :true;
    if(!s) return false;
    if(s->val != t->val) return false;
    return isEqual(s->left,t->left) && isEqual(s->right,t->right);
}
public:
    bool isSubtree(TreeNode* s, TreeNode* t) {
        if(!s && !t) return true;
        if(!s || !t) return false;
        if(s->val == t->val  && isEqual(s,t)) return true;
        return isSubtree(s->left,t) || isSubtree(s->right,t);
​
    }
};