Given two non-empty binary trees s and t, check whether tree t has exactly the same structure and node values with a subtree of s. A subtree of s is a tree consists of a node in s and all of this node's descendants. The tree s could also be considered as a subtree of itself.
 递归实现，代码有点美😄 

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
bool isSametree(struct TreeNode*s,struct TreeNode* t)
{
    if(NULL== s && NULL == t)
        return true;
    if(NULL== s || NULL == t)
        return false;
    if(s->val != t->val)
        return false;
    
    return isSametree(s->left,t->left) && isSametree(s->right,t->right);
}

bool isSubtree(struct TreeNode* s, struct TreeNode* t)
{
    if(NULL == s)
        return false;
    if(isSametree(s,t))
        return true;
    
    return isSubtree(s->left,t) ||isSubtree(s->right,t);

}