LeetCode: Subtree of Another Tree

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.

给两棵非空的二叉树s和t，检查二叉树t是否为s的子树（树t由树s中的一个结点及其所有子孙结点组成）

例如：

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.

解题思路：使用二叉树的先序遍历的思想，对于s，检查以其中每个结点为根节点的子树是否与t相同

代码如下：

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

private boolean isSame(TreeNode s, TreeNode t) {
if (s == null && t == null) return true;
if (s == null || t == null) return false;

if (s.val != t.val) return false;

return isSame(s.left, t.left) && isSame(s.right, t.right);
}
}