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.

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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.

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public class Solution {
    /**
     * @param T1, T2: The roots of binary tree.
     * @return: True if T2 is a subtree of T1, or false.
     */
    public boolean isSubtree(TreeNode T1, TreeNode T2) {
        // write your code here
        if(T2 == null) return true;
        else if(T1 == null) return false;
        else return isSameTree(T1, T2) || isSubtree(T1.left, T2) || isSubtree(T1.right, T2);
    }
   
    private boolean 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);
    }
}