Subtree

You have two every large binary trees: 

T1

, with millions
of nodes, and 

T2

, with hundreds of nodes. Create an algorithm
to decide if 

T2

 is a subtree of 

T1

.

Have you met this question in a real interview? 

Yes

Example

T2 is a subtree of T1 in the following case:

1                3
/ \              /
T1 = 2   3      T2 =  4
/
4

T2 isn't a subtree of T1 in the following case:

1               3
/ \               \
T1 = 2   3       T2 =    4
/
4

Note

A tree T2 is a subtree of T1 if there exists a node n in T1 such that the subtree of n is identical to T2. That is, if you cut off the tree at node n, the two trees would be identical.
题目实际上是sameTree的变形，树T2如果是另外树T1的子树，那么在T1中一定可以找到一颗子树与T2一样，那就是说，我们可以递归的检查，如果T1->val == T2->val，则应该检查isSameTree(T1, T2);如果不相等，则还应该继续递归检查isSubTree(T1->left, T2) || is SubTree(T1->right, T2)。应该属于常规的树的问题，注意各个细节的考虑。/**
* Definition of TreeNode:
* class TreeNode {
* public:
* int val;
* TreeNode *left, *right;
* TreeNode(int val) {
* this->val = val;
* this->left = this->right = NULL;
* }
* }
*/
class Solution {
public:
/**
* @param T1, T2: The roots of binary tree.
* @return: True if T2 is a subtree of T1, or false.
*/
bool isSameTree(TreeNode *T1, TreeNode *T2)
{
if(T1 == NULL && T2 == NULL)
return true;
if(T1 == NULL || T2 == NULL)
return false;
if(T1->val != T2->val)
return false;
return isSameTree(T1->left, T2->left) && isSameTree(T1->right, T2->right);
}

bool isSubtree(TreeNode *T1, TreeNode *T2) {
// write your code here
if(T2 == NULL)
return true;
if(T1 == NULL)
return false;

if(T1->val == T2->val && isSameTree(T1, T2))
return true;

return isSubtree(T1->left, T2) || isSubtree(T1->right, T2);
}
};