Subtree of Another Tree:判断一棵树是否是另一棵树的子树

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.

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.
思路：标准的递归查询，判断本棵树是否与另一棵树相同，不同的话还需判断左右子树是否相同，递归下去。

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isSame(TreeNode s,TreeNode t){
if(s==null&&t==null) return true;
if((s!=null&&t==null)||(s==null&&t!=null)) return false;
if(s.val!=t.val) {
return false;
}else{
return isSame(s.left,t.left)&&isSame(s.right,t.right);
}
}

public boolean isSubtree(TreeNode s, TreeNode t) {
if(s==null) return false;
if(t==null) return false;
return isSame(s,t)||isSubtree(s.left,t)||isSubtree(s.right,t);
}
}